app/email_handler.py

"""
Handle the email *forward* and *reply*. phase. There are 3 actors:
- contact: who sends emails to alias@sl.co address
- SL email handler (this script)
- user personal email: to be protected. Should never leak to contact.

This script makes sure that in the forward phase, the email that is forwarded to user personal email has the following
envelope and header fields:
Envelope:
    mail from: @contact
    rcpt to: @personal_email
Header:
    From: @contact
    To: alias@sl.co # so user knows this email is sent to alias
    Reply-to: special@sl.co # magic HERE

And in the reply phase:
Envelope:
    mail from: @contact
    rcpt to: @contact

Header:
    From: alias@sl.co # so for contact the email comes from alias. magic HERE
    To: @contact

The special@sl.co allows to hide user personal email when user clicks "Reply" to the forwarded email.
It should contain the following info:
- alias
- @contact


"""
import argparse
import asyncio
import email
import os
import time
import uuid
from email import encoders
from email.message import Message
from email.mime.application import MIMEApplication
from email.mime.multipart import MIMEMultipart
from email.utils import formataddr, make_msgid
from io import BytesIO
from smtplib import SMTP, SMTPRecipientsRefused
from typing import List, Tuple, Optional

import aiosmtpd
import aiospamc
import arrow
import spf
from aiosmtpd.smtp import Envelope
from sqlalchemy.exc import IntegrityError

from app import pgp_utils, s3
from app.alias_utils import try_auto_create
from app.config import (
    EMAIL_DOMAIN,
    POSTFIX_SERVER,
    URL,
    ALIAS_DOMAINS,
    POSTFIX_SUBMISSION_TLS,
    UNSUBSCRIBER,
    LOAD_PGP_EMAIL_HANDLER,
    ENFORCE_SPF,
    ALERT_REVERSE_ALIAS_UNKNOWN_MAILBOX,
    ALERT_BOUNCE_EMAIL,
    ALERT_SPAM_EMAIL,
    ALERT_SPF,
    POSTFIX_PORT,
    SENDER,
    SENDER_DIR,
    SPAMASSASSIN_HOST,
    MAX_SPAM_SCORE,
    MAX_REPLY_PHASE_SPAM_SCORE,
    ALERT_SEND_EMAIL_CYCLE,
    ALERT_MAILBOX_IS_ALIAS,
)
from app.email_utils import (
    send_email,
    add_dkim_signature,
    add_or_replace_header,
    delete_header,
    email_belongs_to_alias_domains,
    render,
    get_orig_message_from_bounce,
    delete_all_headers_except,
    get_addrs_from_header,
    get_spam_info,
    get_orig_message_from_spamassassin_report,
    parseaddr_unicode,
    send_email_with_rate_control,
    get_email_domain_part,
    copy,
    to_bytes,
    get_header_from_bounce,
    send_email_at_most_times,
)
from app.extensions import db
from app.greylisting import greylisting_needed
from app.log import LOG
from app.models import (
    Alias,
    Contact,
    EmailLog,
    CustomDomain,
    User,
    RefusedEmail,
    Mailbox,
)
from app.pgp_utils import PGPException
from app.utils import random_string
from init_app import load_pgp_public_keys
from server import create_app, create_light_app

# forward or reply
_DIRECTION = "X-SimpleLogin-Type"

_IP_HEADER = "X-SimpleLogin-Client-IP"
_MAILBOX_ID_HEADER = "X-SimpleLogin-Mailbox-ID"
_EMAIL_LOG_ID_HEADER = "X-SimpleLogin-EmailLog-ID"
_MESSAGE_ID = "Message-ID"


# fix the database connection leak issue
# use this method instead of create_app
def new_app():
    app = create_light_app()

    @app.teardown_appcontext
    def shutdown_session(response_or_exc):
        # same as shutdown_session() in flask-sqlalchemy but this is not enough
        db.session.remove()

        # dispose the engine too
        db.engine.dispose()

    return app


def get_or_create_contact(
    contact_from_header: str, mail_from: str, alias: Alias
) -> Contact:
    """
    contact_from_header is the RFC 2047 format FROM header
    """
    # contact_from_header can be None, use mail_from in this case instead
    contact_from_header = contact_from_header or mail_from

    # force convert header to string, sometimes contact_from_header is Header object
    contact_from_header = str(contact_from_header)

    contact_name, contact_email = parseaddr_unicode(contact_from_header)
    if not contact_email:
        # From header is wrongly formatted, try with mail_from
        LOG.warning("From header is empty, parse mail_from %s %s", mail_from, alias)
        contact_name, contact_email = parseaddr_unicode(mail_from)
        if not contact_email:
            LOG.exception(
                "Cannot parse contact from from_header:%s, mail_from:%s",
                contact_from_header,
                mail_from,
            )

    contact = Contact.get_by(alias_id=alias.id, website_email=contact_email)
    if contact:
        if contact.name != contact_name:
            LOG.d(
                "Update contact %s name %s to %s",
                contact,
                contact.name,
                contact_name,
            )
            contact.name = contact_name
            db.session.commit()
    else:
        LOG.debug(
            "create contact for alias %s and contact %s",
            alias,
            contact_from_header,
        )

        reply_email = generate_reply_email()

        try:
            contact = Contact.create(
                user_id=alias.user_id,
                alias_id=alias.id,
                website_email=contact_email,
                name=contact_name,
                reply_email=reply_email,
                mail_from=mail_from,
                from_header=contact_from_header,
            )
            db.session.commit()
        except IntegrityError:
            LOG.warning("Contact %s %s already exist", alias, contact_email)
            db.session.rollback()
            contact = Contact.get_by(alias_id=alias.id, website_email=contact_email)

    return contact


def replace_header_when_forward(msg: Message, alias: Alias, header: str):
    """
    Replace CC or To header by Reply emails in forward phase
    """
    addrs = get_addrs_from_header(msg, header)

    # Nothing to do
    if not addrs:
        return

    new_addrs: [str] = []

    for addr in addrs:
        contact_name, contact_email = parseaddr_unicode(addr)

        # no transformation when alias is already in the header
        if contact_email == alias.email:
            new_addrs.append(addr)
            continue

        contact = Contact.get_by(alias_id=alias.id, website_email=contact_email)
        if contact:
            # update the contact name if needed
            if contact.name != contact_name:
                LOG.d(
                    "Update contact %s name %s to %s",
                    contact,
                    contact.name,
                    contact_name,
                )
                contact.name = contact_name
                db.session.commit()
        else:
            LOG.debug(
                "create contact for alias %s and email %s, header %s",
                alias,
                contact_email,
                header,
            )

            reply_email = generate_reply_email()

            try:
                contact = Contact.create(
                    user_id=alias.user_id,
                    alias_id=alias.id,
                    website_email=contact_email,
                    name=contact_name,
                    reply_email=reply_email,
                    is_cc=header.lower() == "cc",
                    from_header=addr,
                )
                db.session.commit()
            except IntegrityError:
                LOG.warning("Contact %s %s already exist", alias, contact_email)
                db.session.rollback()
                contact = Contact.get_by(alias_id=alias.id, website_email=contact_email)

        new_addrs.append(contact.new_addr())

    if new_addrs:
        new_header = ",".join(new_addrs)
        LOG.d("Replace %s header, old: %s, new: %s", header, msg[header], new_header)
        add_or_replace_header(msg, header, new_header)
    else:
        LOG.d("Delete %s header, old value %s", header, msg[header])
        delete_header(msg, header)


def replace_header_when_reply(msg: Message, alias: Alias, header: str):
    """
    Replace CC or To Reply emails by original emails
    """
    addrs = get_addrs_from_header(msg, header)

    # Nothing to do
    if not addrs:
        return

    new_addrs: [str] = []

    for addr in addrs:
        _, reply_email = parseaddr_unicode(addr)

        # no transformation when alias is already in the header
        if reply_email == alias.email:
            continue

        contact = Contact.get_by(reply_email=reply_email)
        if not contact:
            LOG.warning(
                "%s email in reply phase %s must be reply emails", header, reply_email
            )
            # still keep this email in header
            new_addrs.append(addr)
        else:
            new_addrs.append(formataddr((contact.name, contact.website_email)))

    if new_addrs:
        new_header = ",".join(new_addrs)
        LOG.d("Replace %s header, old: %s, new: %s", header, msg[header], new_header)
        add_or_replace_header(msg, header, new_header)
    else:
        LOG.d("delete the %s header. Old value %s", header, msg[header])
        delete_header(msg, header)


def replace_str_in_msg(msg: Message, fr: str, to: str):
    if msg.get_content_maintype() != "text":
        return msg
    new_body = msg.get_payload(decode=True).replace(fr.encode(), to.encode())

    # If utf-8 decoding fails, do not touch message part
    try:
        new_body = new_body.decode("utf-8")
    except:
        return msg

    cte = (
        msg["Content-Transfer-Encoding"].lower()
        if msg["Content-Transfer-Encoding"]
        else None
    )
    subtype = msg.get_content_subtype()
    delete_header(msg, "Content-Transfer-Encoding")
    delete_header(msg, "Content-Type")

    email.contentmanager.set_text_content(msg, new_body, subtype=subtype, cte=cte)
    return msg


def generate_reply_email():
    # generate a reply_email, make sure it is unique
    # not use while loop to avoid infinite loop
    reply_email = f"reply+{random_string(30)}@{EMAIL_DOMAIN}"
    for _ in range(1000):
        if not Contact.get_by(reply_email=reply_email):
            # found!
            break
        reply_email = f"reply+{random_string(30)}@{EMAIL_DOMAIN}"

    return reply_email


def should_append_alias(msg: Message, address: str):
    """whether an alias should be appended to TO header in message"""

    # # force convert header to string, sometimes addrs is Header object
    if msg["To"] and address.lower() in str(msg["To"]).lower():
        return False
    if msg["Cc"] and address.lower() in str(msg["Cc"]).lower():
        return False

    return True


_MIME_HEADERS = [
    "MIME-Version",
    "Content-Type",
    "Content-Disposition",
    "Content-Transfer-Encoding",
]
_MIME_HEADERS = [h.lower() for h in _MIME_HEADERS]


def prepare_pgp_message(orig_msg: Message, pgp_fingerprint: str):
    msg = MIMEMultipart("encrypted", protocol="application/pgp-encrypted")

    # copy all headers from original message except all standard MIME headers
    for i in reversed(range(len(orig_msg._headers))):
        header_name = orig_msg._headers[i][0].lower()
        if header_name.lower() not in _MIME_HEADERS:
            msg[header_name] = orig_msg._headers[i][1]

    # Delete unnecessary headers in orig_msg except to save space
    delete_all_headers_except(
        orig_msg,
        _MIME_HEADERS,
    )

    first = MIMEApplication(
        _subtype="pgp-encrypted", _encoder=encoders.encode_7or8bit, _data=""
    )
    first.set_payload("Version: 1")
    msg.attach(first)

    second = MIMEApplication(
        "octet-stream", _encoder=encoders.encode_7or8bit, name="encrypted.asc"
    )
    second.add_header("Content-Disposition", 'inline; filename="encrypted.asc"')
    # encrypt original message
    encrypted_data = pgp_utils.encrypt_file(
        BytesIO(orig_msg.as_bytes()), pgp_fingerprint
    )
    second.set_payload(encrypted_data)
    msg.attach(second)

    return msg


def handle_email_sent_to_ourself(alias, mailbox, msg: Message, user):
    # store the refused email
    random_name = str(uuid.uuid4())
    full_report_path = f"refused-emails/cycle-{random_name}.eml"
    s3.upload_email_from_bytesio(full_report_path, BytesIO(msg.as_bytes()), random_name)
    refused_email = RefusedEmail.create(
        path=None, full_report_path=full_report_path, user_id=alias.user_id
    )
    db.session.commit()
    LOG.d("Create refused email %s", refused_email)
    # link available for 6 days as it gets deleted in 7 days
    refused_email_url = refused_email.get_url(expires_in=518400)

    send_email_at_most_times(
        user,
        ALERT_SEND_EMAIL_CYCLE,
        mailbox.email,
        f"Email sent to {alias.email} from its own mailbox {mailbox.email}",
        render(
            "transactional/cycle-email.txt",
            name=user.name or "",
            alias=alias,
            mailbox=mailbox,
            refused_email_url=refused_email_url,
        ),
        render(
            "transactional/cycle-email.html",
            name=user.name or "",
            alias=alias,
            mailbox=mailbox,
            refused_email_url=refused_email_url,
        ),
    )


async def handle_forward(
    envelope, smtp: SMTP, msg: Message, rcpt_to: str
) -> List[Tuple[bool, str]]:
    """return an array of SMTP status (is_success, smtp_status)
    is_success indicates whether an email has been delivered and
    smtp_status is the SMTP Status ("250 Message accepted", "550 Non-existent email address", etc)
    """
    address = rcpt_to  # alias@SL

    alias = Alias.get_by(email=address)
    if not alias:
        LOG.d("alias %s not exist. Try to see if it can be created on the fly", address)
        alias = try_auto_create(address)
        if not alias:
            LOG.d("alias %s cannot be created on-the-fly, return 550", address)
            return [(False, "550 SL E3 Email not exist")]

    mail_from = envelope.mail_from
    for mb in alias.mailboxes:
        # email send from a mailbox to alias
        if mb.email == mail_from:
            LOG.warning("cycle email sent from %s to %s", mb, alias)
            handle_email_sent_to_ourself(alias, mb, msg, alias.user)
            return [(True, "250 Message accepted for delivery")]

    contact = get_or_create_contact(msg["From"], envelope.mail_from, alias)
    email_log = EmailLog.create(contact_id=contact.id, user_id=contact.user_id)
    db.session.commit()

    if not alias.enabled:
        LOG.d("%s is disabled, do not forward", alias)
        email_log.blocked = True

        db.session.commit()
        # do not return 5** to allow user to receive emails later when alias is enabled
        return [(True, "250 Message accepted for delivery")]

    user = alias.user

    ret = []
    mailboxes = alias.mailboxes

    # no valid mailbox
    if not mailboxes:
        return [(False, "550 SL E16 invalid mailbox")]

    # no need to create a copy of message
    if len(mailboxes) == 1:
        mailbox = mailboxes[0]
        if not mailbox.verified:
            LOG.debug("Mailbox %s unverified, do not forward", mailbox)
            return [(False, "550 SL E18 unverified mailbox")]
        else:
            ret.append(
                await forward_email_to_mailbox(
                    alias, msg, email_log, contact, envelope, smtp, mailbox, user
                )
            )
    # create a copy of message for each forward
    else:
        for mailbox in mailboxes:
            if not mailbox.verified:
                LOG.debug("Mailbox %s unverified, do not forward", mailbox)
                ret.append((False, "550 SL E19 unverified mailbox"))
            else:
                ret.append(
                    await forward_email_to_mailbox(
                        alias,
                        copy(msg),
                        email_log,
                        contact,
                        envelope,
                        smtp,
                        mailbox,
                        user,
                    )
                )

    return ret


async def forward_email_to_mailbox(
    alias,
    msg: Message,
    email_log: EmailLog,
    contact: Contact,
    envelope,
    smtp: SMTP,
    mailbox,
    user,
) -> (bool, str):
    LOG.d("Forward %s -> %s -> %s", contact, alias, mailbox)

    # sanity check: make sure mailbox is not actually an alias
    if get_email_domain_part(alias.email) == get_email_domain_part(mailbox.email):
        LOG.warning(
            "Mailbox has the same domain as alias. %s -> %s -> %s",
            contact,
            alias,
            mailbox,
        )
        mailbox_url = f"{URL}/dashboard/mailbox/{mailbox.id}/"
        send_email_with_rate_control(
            user,
            ALERT_MAILBOX_IS_ALIAS,
            user.email,
            f"Your SimpleLogin mailbox {mailbox.email} cannot be an email alias",
            render(
                "transactional/mailbox-invalid.txt",
                name=user.name or "",
                mailbox=mailbox,
                mailbox_url=mailbox_url,
            ),
            render(
                "transactional/mailbox-invalid.html",
                name=user.name or "",
                mailbox=mailbox,
                mailbox_url=mailbox_url,
            ),
            max_nb_alert=1,
        )

        # retry later
        # so when user fixes the mailbox, the email can be delivered
        return False, "421 SL E14"

    # Spam check
    spam_status = ""
    is_spam = False

    if SPAMASSASSIN_HOST:
        start = time.time()
        spam_score = await get_spam_score(msg)
        LOG.d(
            "%s -> %s - spam score %s in %s seconds",
            contact,
            alias,
            spam_score,
            time.time() - start,
        )
        email_log.spam_score = spam_score
        db.session.commit()

        if (user.max_spam_score and spam_score > user.max_spam_score) or (
            not user.max_spam_score and spam_score > MAX_SPAM_SCORE
        ):
            is_spam = True
            spam_status = "Spam detected by SpamAssassin server"
    else:
        is_spam, spam_status = get_spam_info(msg, max_score=user.max_spam_score)

    if is_spam:
        LOG.warning("Email detected as spam. Alias: %s, from: %s", alias, contact)
        email_log.is_spam = True
        email_log.spam_status = spam_status
        db.session.commit()

        handle_spam(contact, alias, msg, user, mailbox, email_log)
        return False, "550 SL E1 Email detected as spam"

    # create PGP email if needed
    if mailbox.pgp_finger_print and user.is_premium() and not alias.disable_pgp:
        LOG.d("Encrypt message using mailbox %s", mailbox)
        try:
            msg = prepare_pgp_message(msg, mailbox.pgp_finger_print)
        except PGPException:
            LOG.exception(
                "Cannot encrypt message %s -> %s. %s %s", contact, alias, mailbox, user
            )
            # so the client can retry later
            return False, "421 SL E12 Retry later"

    # add custom header
    add_or_replace_header(msg, _DIRECTION, "Forward")

    # remove reply-to & sender header if present
    delete_header(msg, "Reply-To")
    delete_header(msg, "Sender")

    delete_header(msg, _IP_HEADER)
    add_or_replace_header(msg, _MAILBOX_ID_HEADER, str(mailbox.id))
    add_or_replace_header(msg, _EMAIL_LOG_ID_HEADER, str(email_log.id))
    add_or_replace_header(msg, _MESSAGE_ID, make_msgid(str(email_log.id), EMAIL_DOMAIN))

    # change the from header so the sender comes from @SL
    # so it can pass DMARC check
    # replace the email part in from: header
    contact_from_header = msg["From"]
    new_from_header = contact.new_addr()
    add_or_replace_header(msg, "From", new_from_header)
    LOG.d("new_from_header:%s, old header %s", new_from_header, contact_from_header)

    # replace CC & To emails by reply-email for all emails that are not alias
    replace_header_when_forward(msg, alias, "Cc")
    replace_header_when_forward(msg, alias, "To")

    # append alias into the TO header if it's not present in To or CC
    if should_append_alias(msg, alias.email):
        LOG.d("append alias %s  to TO header %s", alias, msg["To"])
        if msg["To"]:
            to_header = msg["To"] + "," + alias.email
        else:
            to_header = alias.email

        add_or_replace_header(msg, "To", to_header.strip())

    # add List-Unsubscribe header
    if UNSUBSCRIBER:
        unsubscribe_link = f"mailto:{UNSUBSCRIBER}?subject={alias.id}="
        add_or_replace_header(msg, "List-Unsubscribe", f"<{unsubscribe_link}>")
    else:
        unsubscribe_link = f"{URL}/dashboard/unsubscribe/{alias.id}"
        add_or_replace_header(msg, "List-Unsubscribe", f"<{unsubscribe_link}>")
        add_or_replace_header(
            msg, "List-Unsubscribe-Post", "List-Unsubscribe=One-Click"
        )

    add_dkim_signature(msg, EMAIL_DOMAIN)

    LOG.d(
        "Forward mail from %s to %s, mail_options %s, rcpt_options %s ",
        contact.website_email,
        mailbox.email,
        envelope.mail_options,
        envelope.rcpt_options,
    )

    # smtp.send_message has UnicodeEncodeErroremail issue
    # encode message raw directly instead
    try:
        smtp.sendmail(
            contact.reply_email,
            mailbox.email,
            msg.as_bytes(),
            envelope.mail_options,
            envelope.rcpt_options,
        )
    except SMTPRecipientsRefused:
        # that means the mailbox is maybe invalid
        LOG.warning(
            "SMTPRecipientsRefused forward phase %s -> %s -> %s",
            contact,
            alias,
            mailbox,
        )
        # return 421 so Postfix can retry later
        return False, "421 SL E17 Retry later"
    else:
        db.session.commit()
        return True, "250 Message accepted for delivery"


async def handle_reply(envelope, smtp: SMTP, msg: Message, rcpt_to: str) -> (bool, str):
    """
    return whether an email has been delivered and
    the smtp status ("250 Message accepted", "550 Non-existent email address", etc)
    """
    reply_email = rcpt_to

    # reply_email must end with EMAIL_DOMAIN
    if not reply_email.endswith(EMAIL_DOMAIN):
        LOG.warning(f"Reply email {reply_email} has wrong domain")
        return False, "550 SL E2"

    contact = Contact.get_by(reply_email=reply_email)
    if not contact:
        LOG.warning(f"No such forward-email with {reply_email} as reply-email")
        return False, "550 SL E4 Email not exist"

    alias = contact.alias
    address: str = contact.alias.email
    alias_domain = address[address.find("@") + 1 :]

    # alias must end with one of the ALIAS_DOMAINS or custom-domain
    if not email_belongs_to_alias_domains(alias.email):
        if not CustomDomain.get_by(domain=alias_domain):
            return False, "550 SL E5"

    user = alias.user
    mail_from = envelope.mail_from

    # bounce email initiated by Postfix
    # can happen in case emails cannot be delivered to user-email
    # in this case Postfix will try to send a bounce report to original sender, which is
    # the "reply email"
    if mail_from == "<>":
        LOG.warning(
            "Bounce when sending to alias %s from %s, user %s",
            alias,
            contact,
            user,
        )

        handle_bounce(contact, alias, msg, user)
        return False, "550 SL E6"

    # Anti-spoofing
    mailbox = get_mailbox_from_mail_from(mail_from, alias)
    if not mailbox:
        if alias.disable_email_spoofing_check:
            # ignore this error, use default alias mailbox
            LOG.warning(
                "ignore unknown sender to reverse-alias %s: %s -> %s",
                mail_from,
                alias,
                contact,
            )
            mailbox = alias.mailbox
        else:
            # only mailbox can send email to the reply-email
            handle_unknown_mailbox(envelope, msg, reply_email, user, alias, contact)
            return False, "550 SL E7"

    if ENFORCE_SPF and mailbox.force_spf and not alias.disable_email_spoofing_check:
        ip = msg[_IP_HEADER]
        if not spf_pass(ip, envelope, mailbox, user, alias, contact.website_email, msg):
            # cannot use 4** here as sender will retry. 5** because that generates bounce report
            return True, "250 SL E11"

    email_log = EmailLog.create(
        contact_id=contact.id, is_reply=True, user_id=contact.user_id
    )

    # Spam check
    spam_status = ""
    is_spam = False

    # do not use user.max_spam_score here
    if SPAMASSASSIN_HOST:
        start = time.time()
        spam_score = await get_spam_score(msg)
        LOG.d(
            "%s -> %s - spam score %s in %s seconds",
            alias,
            contact,
            spam_score,
            time.time() - start,
        )
        email_log.spam_score = spam_score
        if spam_score > MAX_REPLY_PHASE_SPAM_SCORE:
            is_spam = True
            spam_status = "Spam detected by SpamAssassin server"
    else:
        is_spam, spam_status = get_spam_info(msg, max_score=MAX_REPLY_PHASE_SPAM_SCORE)

    if is_spam:
        LOG.exception(
            "Reply phase - email sent from %s to %s detected as spam", alias, contact
        )

        email_log.is_spam = True
        email_log.spam_status = spam_status
        db.session.commit()

        handle_spam(contact, alias, msg, user, mailbox, email_log, is_reply=True)
        return False, "550 SL E15 Email detected as spam"

    delete_header(msg, _IP_HEADER)

    delete_header(msg, "DKIM-Signature")
    delete_header(msg, "Received")

    # make the email comes from alias
    from_header = alias.email
    # add alias name from alias
    if alias.name:
        LOG.d("Put alias name in from header")
        from_header = formataddr((alias.name, alias.email))
    elif alias.custom_domain:
        LOG.d("Put domain default alias name in from header")

        # add alias name from domain
        if alias.custom_domain.name:
            from_header = formataddr((alias.custom_domain.name, alias.email))

    add_or_replace_header(msg, "From", from_header)

    # some email providers like ProtonMail adds automatically the Reply-To field
    # make sure to delete it
    delete_header(msg, "Reply-To")

    # remove sender header if present as this could reveal user real email
    delete_header(msg, "Sender")
    delete_header(msg, "X-Sender")

    replace_header_when_reply(msg, alias, "To")
    replace_header_when_reply(msg, alias, "Cc")

    add_or_replace_header(
        msg,
        _MESSAGE_ID,
        make_msgid(str(email_log.id), get_email_domain_part(alias.email)),
    )
    add_or_replace_header(msg, _EMAIL_LOG_ID_HEADER, str(email_log.id))

    add_or_replace_header(msg, _DIRECTION, "Reply")

    # Received-SPF is injected by postfix-policyd-spf-python can reveal user original email
    delete_header(msg, "Received-SPF")

    LOG.d(
        "send email from %s to %s, mail_options:%s,rcpt_options:%s",
        alias.email,
        contact.website_email,
        envelope.mail_options,
        envelope.rcpt_options,
    )

    # replace "ra+string@simplelogin.co" by the contact email in the email body
    # as this is usually included when replying
    if user.replace_reverse_alias:
        if msg.is_multipart():
            for part in msg.walk():
                if part.get_content_maintype() != "text":
                    continue
                part = replace_str_in_msg(part, reply_email, contact.website_email)

        else:
            msg = replace_str_in_msg(msg, reply_email, contact.website_email)

    if alias_domain in ALIAS_DOMAINS:
        add_dkim_signature(msg, alias_domain)
    # add DKIM-Signature for custom-domain alias
    else:
        custom_domain: CustomDomain = CustomDomain.get_by(domain=alias_domain)
        if custom_domain.dkim_verified:
            add_dkim_signature(msg, alias_domain)

    # create PGP email if needed
    if contact.pgp_finger_print and user.is_premium():
        LOG.d("Encrypt message for contact %s", contact)
        try:
            msg = prepare_pgp_message(msg, contact.pgp_finger_print)
        except PGPException:
            LOG.exception(
                "Cannot encrypt message %s -> %s. %s %s", alias, contact, mailbox, user
            )
            # to not save the email_log
            db.session.rollback()
            # return 421 so the client can retry later
            return False, "421 SL E13 Retry later"

    try:
        smtp.sendmail(
            alias.email,
            contact.website_email,
            msg.as_bytes(),
            envelope.mail_options,
            envelope.rcpt_options,
        )
    except Exception:
        # to not save the email_log
        db.session.rollback()

        LOG.warning("Cannot send email from %s to %s", alias, contact)
        send_email(
            mailbox.email,
            f"Email cannot be sent to {contact.email} from {alias.email}",
            render(
                "transactional/reply-error.txt",
                user=user,
                alias=alias,
                contact=contact,
                contact_domain=get_email_domain_part(contact.email),
            ),
            render(
                "transactional/reply-error.html",
                user=user,
                alias=alias,
                contact=contact,
                contact_domain=get_email_domain_part(contact.email),
            ),
        )

    # return 250 even if error as user is already informed of the incident and can retry sending the email
    db.session.commit()
    return True, "250 Message accepted for delivery"


def get_mailbox_from_mail_from(mail_from: str, alias) -> Optional[Mailbox]:
    """return the corresponding mailbox given the mail_from and alias
    Usually the mail_from=mailbox.email but it can also be one of the authorized address
    """
    for mailbox in alias.mailboxes:
        if mailbox.email == mail_from:
            return mailbox

        for address in mailbox.authorized_addresses:
            if address.email == mail_from:
                LOG.debug(
                    "Found an authorized address for %s %s %s", alias, mailbox, address
                )
                return mailbox

    return None


def spf_pass(
    ip: str,
    envelope,
    mailbox: Mailbox,
    user: User,
    alias: Alias,
    contact_email: str,
    msg: Message,
) -> bool:
    if ip:
        LOG.d("Enforce SPF")
        try:
            r = spf.check2(i=ip, s=envelope.mail_from, h=None)
        except Exception:
            LOG.exception("SPF error, mailbox %s, ip %s", mailbox.email, ip)
        else:
            # TODO: Handle temperr case (e.g. dns timeout)
            # only an absolute pass, or no SPF policy at all is 'valid'
            if r[0] not in ["pass", "none"]:
                LOG.warning(
                    "SPF fail for mailbox %s, reason %s, failed IP %s",
                    mailbox.email,
                    r[0],
                    ip,
                )
                send_email_with_rate_control(
                    user,
                    ALERT_SPF,
                    mailbox.email,
                    f"SimpleLogin Alert: attempt to send emails from your alias {alias.email} from unknown IP Address",
                    render(
                        "transactional/spf-fail.txt",
                        name=user.name,
                        alias=alias.email,
                        ip=ip,
                        mailbox_url=URL + f"/dashboard/mailbox/{mailbox.id}#spf",
                        to_email=contact_email,
                        subject=msg["Subject"],
                        time=arrow.now(),
                    ),
                    render(
                        "transactional/spf-fail.html",
                        name=user.name,
                        alias=alias.email,
                        ip=ip,
                        mailbox_url=URL + f"/dashboard/mailbox/{mailbox.id}#spf",
                        to_email=contact_email,
                        subject=msg["Subject"],
                        time=arrow.now(),
                    ),
                )
                return False

    else:
        LOG.warning(
            "Could not find %s header %s -> %s",
            _IP_HEADER,
            mailbox.email,
            contact_email,
        )

    return True


def handle_unknown_mailbox(
    envelope, msg, reply_email: str, user: User, alias: Alias, contact: Contact
):
    LOG.warning(
        f"Reply email can only be used by mailbox. "
        f"Actual mail_from: %s. msg from header: %s, reverse-alias %s, %s %s %s",
        envelope.mail_from,
        msg["From"],
        reply_email,
        alias,
        user,
        contact,
    )

    authorize_address_link = (
        f"{URL}/dashboard/mailbox/{alias.mailbox_id}/#authorized-address"
    )
    send_email_with_rate_control(
        user,
        ALERT_REVERSE_ALIAS_UNKNOWN_MAILBOX,
        user.email,
        f"Attempt to use your alias {alias.email} from {envelope.mail_from}",
        render(
            "transactional/reply-must-use-personal-email.txt",
            name=user.name,
            alias=alias,
            sender=envelope.mail_from,
            authorize_address_link=authorize_address_link,
        ),
        render(
            "transactional/reply-must-use-personal-email.html",
            name=user.name,
            alias=alias,
            sender=envelope.mail_from,
            authorize_address_link=authorize_address_link,
        ),
    )

    # Notify sender that they cannot send emails to this address
    send_email_with_rate_control(
        user,
        ALERT_REVERSE_ALIAS_UNKNOWN_MAILBOX,
        envelope.mail_from,
        f"Your email ({envelope.mail_from}) is not allowed to send emails to {reply_email}",
        render(
            "transactional/send-from-alias-from-unknown-sender.txt",
            sender=envelope.mail_from,
            reply_email=reply_email,
        ),
        render(
            "transactional/send-from-alias-from-unknown-sender.html",
            sender=envelope.mail_from,
            reply_email=reply_email,
        ),
    )


def handle_bounce(contact: Contact, alias: Alias, msg: Message, user: User):
    disable_alias_link = f"{URL}/dashboard/unsubscribe/{alias.id}"

    # Store the bounced email
    # generate a name for the email
    random_name = str(uuid.uuid4())

    full_report_path = f"refused-emails/full-{random_name}.eml"
    s3.upload_email_from_bytesio(full_report_path, BytesIO(msg.as_bytes()), random_name)

    file_path = None
    mailbox = None
    email_log: EmailLog = None
    orig_msg = get_orig_message_from_bounce(msg)
    if not orig_msg:
        # Some MTA does not return the original message in bounce message
        # nothing we can do here
        LOG.warning(
            "Cannot parse original message from bounce message %s %s %s %s",
            alias,
            user,
            contact,
            full_report_path,
        )
    else:
        file_path = f"refused-emails/{random_name}.eml"
        s3.upload_email_from_bytesio(
            file_path, BytesIO(orig_msg.as_bytes()), random_name
        )
        try:
            mailbox_id = int(orig_msg[_MAILBOX_ID_HEADER])
        except TypeError:
            LOG.warning(
                "cannot parse mailbox from original message header %s",
                orig_msg[_MAILBOX_ID_HEADER],
            )
        else:
            mailbox = Mailbox.get(mailbox_id)
            if not mailbox or mailbox.user_id != user.id:
                LOG.exception(
                    "Tampered message mailbox_id %s, %s, %s, %s %s",
                    mailbox_id,
                    user,
                    alias,
                    contact,
                    full_report_path,
                )
                # cannot use this tampered mailbox, reset it
                mailbox = None

        # try to get the original email_log
        try:
            email_log_id = int(orig_msg[_EMAIL_LOG_ID_HEADER])
        except TypeError:
            LOG.warning(
                "cannot parse email log from original message header %s",
                orig_msg[_EMAIL_LOG_ID_HEADER],
            )
        else:
            email_log = EmailLog.get(email_log_id)

    refused_email = RefusedEmail.create(
        path=file_path, full_report_path=full_report_path, user_id=user.id
    )
    db.session.flush()
    LOG.d("Create refused email %s", refused_email)

    if not mailbox:
        LOG.debug("Try to get mailbox from bounce report")
        try:
            mailbox_id = int(get_header_from_bounce(msg, _MAILBOX_ID_HEADER))
        except Exception:
            LOG.warning("cannot get mailbox-id from bounce report, %s", refused_email)
        else:
            mailbox = Mailbox.get(mailbox_id)
            if not mailbox or mailbox.user_id != user.id:
                LOG.exception(
                    "Tampered message mailbox_id %s, %s, %s, %s %s",
                    mailbox_id,
                    user,
                    alias,
                    contact,
                    full_report_path,
                )
                mailbox = None

    if not email_log:
        LOG.d("Try to get email log from bounce report")
        try:
            email_log_id = int(get_header_from_bounce(msg, _EMAIL_LOG_ID_HEADER))
        except Exception:
            LOG.warning("cannot get email log id from bounce report, %s", refused_email)
        else:
            email_log = EmailLog.get(email_log_id)

    # use the default mailbox as the last option
    if not mailbox:
        LOG.warning("Use %s default mailbox %s", alias, refused_email)
        mailbox = alias.mailbox

    # create a new email log as the last option
    if not email_log:
        LOG.warning("cannot get the original email_log, create a new one")
        email_log: EmailLog = EmailLog.create(
            contact_id=contact.id, user_id=contact.user_id
        )

    email_log.bounced = True
    email_log.refused_email_id = refused_email.id
    email_log.bounced_mailbox_id = mailbox.id
    db.session.commit()

    refused_email_url = (
        URL + f"/dashboard/refused_email?highlight_id=" + str(email_log.id)
    )

    nb_bounced = EmailLog.filter_by(contact_id=contact.id, bounced=True).count()
    if nb_bounced >= 2 and alias.cannot_be_disabled:
        LOG.warning("%s cannot be disabled", alias)

    # inform user if this is the first bounced email
    if nb_bounced == 1 or (nb_bounced >= 2 and alias.cannot_be_disabled):
        LOG.d(
            "Inform user %s about bounced email sent by %s to alias %s",
            user,
            contact.website_email,
            alias,
        )
        send_email_with_rate_control(
            user,
            ALERT_BOUNCE_EMAIL,
            user.email,
            f"Email from {contact.website_email} to {alias.email} cannot be delivered to your inbox",
            render(
                "transactional/bounced-email.txt",
                name=user.name,
                alias=alias,
                website_email=contact.website_email,
                disable_alias_link=disable_alias_link,
                refused_email_url=refused_email_url,
                mailbox_email=mailbox.email,
            ),
            render(
                "transactional/bounced-email.html",
                name=user.name,
                alias=alias,
                website_email=contact.website_email,
                disable_alias_link=disable_alias_link,
                refused_email_url=refused_email_url,
                mailbox_email=mailbox.email,
            ),
        )
    # disable the alias the second time email is bounced
    elif nb_bounced >= 2:
        LOG.d(
            "Bounce happens again with alias %s from %s. Disable alias now ",
            alias,
            contact.website_email,
        )
        alias.enabled = False
        db.session.commit()

        send_email_with_rate_control(
            user,
            ALERT_BOUNCE_EMAIL,
            user.email,
            f"Alias {alias.email} has been disabled due to second undelivered email from {contact.website_email}",
            render(
                "transactional/automatic-disable-alias.txt",
                name=user.name,
                alias=alias,
                website_email=contact.website_email,
                refused_email_url=refused_email_url,
                mailbox_email=mailbox.email,
            ),
            render(
                "transactional/automatic-disable-alias.html",
                name=user.name,
                alias=alias,
                website_email=contact.website_email,
                refused_email_url=refused_email_url,
                mailbox_email=mailbox.email,
            ),
        )


def handle_spam(
    contact: Contact,
    alias: Alias,
    msg: Message,
    user: User,
    mailbox: Mailbox,
    email_log: EmailLog,
    is_reply=False,  # whether the email is in forward or reply phase
):
    # Store the report & original email
    orig_msg = get_orig_message_from_spamassassin_report(msg)
    # generate a name for the email
    random_name = str(uuid.uuid4())

    full_report_path = f"spams/full-{random_name}.eml"
    s3.upload_email_from_bytesio(full_report_path, BytesIO(msg.as_bytes()), random_name)

    file_path = None
    if orig_msg:
        file_path = f"spams/{random_name}.eml"
        s3.upload_email_from_bytesio(
            file_path, BytesIO(orig_msg.as_bytes()), random_name
        )

    refused_email = RefusedEmail.create(
        path=file_path, full_report_path=full_report_path, user_id=user.id
    )
    db.session.flush()

    email_log.refused_email_id = refused_email.id
    db.session.commit()

    LOG.d("Create spam email %s", refused_email)

    refused_email_url = (
        URL + f"/dashboard/refused_email?highlight_id=" + str(email_log.id)
    )
    disable_alias_link = f"{URL}/dashboard/unsubscribe/{alias.id}"

    if is_reply:
        LOG.d(
            "Inform %s (%s) about spam email sent from alias %s to %s",
            mailbox,
            user,
            alias,
            contact,
        )
        send_email_with_rate_control(
            user,
            ALERT_SPAM_EMAIL,
            mailbox.email,
            f"Email from {contact.website_email} to {alias.email} is detected as spam",
            render(
                "transactional/spam-email-reply-phase.txt",
                name=user.name,
                alias=alias,
                website_email=contact.website_email,
                disable_alias_link=disable_alias_link,
                refused_email_url=refused_email_url,
            ),
            render(
                "transactional/spam-email-reply-phase.html",
                name=user.name,
                alias=alias,
                website_email=contact.website_email,
                disable_alias_link=disable_alias_link,
                refused_email_url=refused_email_url,
            ),
        )
    else:
        # inform user
        LOG.d(
            "Inform %s (%s) about spam email sent by %s to alias %s",
            mailbox,
            user,
            contact,
            alias,
        )
        send_email_with_rate_control(
            user,
            ALERT_SPAM_EMAIL,
            mailbox.email,
            f"Email from {contact.website_email} to {alias.email} is detected as spam",
            render(
                "transactional/spam-email.txt",
                name=user.name,
                alias=alias,
                website_email=contact.website_email,
                disable_alias_link=disable_alias_link,
                refused_email_url=refused_email_url,
            ),
            render(
                "transactional/spam-email.html",
                name=user.name,
                alias=alias,
                website_email=contact.website_email,
                disable_alias_link=disable_alias_link,
                refused_email_url=refused_email_url,
            ),
        )


def handle_unsubscribe(envelope: Envelope):
    msg = email.message_from_bytes(envelope.original_content)

    # format: alias_id:
    subject = msg["Subject"]
    try:
        # subject has the format {alias.id}=
        if subject.endswith("="):
            alias_id = int(subject[:-1])
        # some email providers might strip off the = suffix
        else:
            alias_id = int(subject)

        alias = Alias.get(alias_id)
    except Exception:
        LOG.warning("Cannot parse alias from subject %s", msg["Subject"])
        return "550 SL E8 Wrongly formatted subject"

    if not alias:
        LOG.warning("No such alias %s", alias_id)
        return "550 SL E9 Email not exist"

    # This sender cannot unsubscribe
    mail_from = envelope.mail_from
    mailbox = Mailbox.get_by(user_id=alias.user_id, email=mail_from)
    if not mailbox or mailbox not in alias.mailboxes:
        LOG.d("%s cannot disable alias %s", envelope.mail_from, alias)
        return "550 SL E10 unauthorized"

    # Sender is owner of this alias
    alias.enabled = False
    db.session.commit()
    user = alias.user

    enable_alias_url = URL + f"/dashboard/?highlight_alias_id={alias.id}"
    for mailbox in alias.mailboxes:
        send_email(
            mailbox.email,
            f"Alias {alias.email} has been disabled successfully",
            render(
                "transactional/unsubscribe-disable-alias.txt",
                user=user,
                alias=alias.email,
                enable_alias_url=enable_alias_url,
            ),
            render(
                "transactional/unsubscribe-disable-alias.html",
                user=user,
                alias=alias.email,
                enable_alias_url=enable_alias_url,
            ),
        )

    return "250 Unsubscribe request accepted"


def handle_sender_email(envelope: Envelope):
    filename = (
        arrow.now().format("YYYY-MM-DD_HH-mm-ss") + "_" + random_string(10) + ".eml"
    )
    filepath = os.path.join(SENDER_DIR, filename)

    with open(filepath, "wb") as f:
        f.write(envelope.original_content)

    LOG.d("Write email to sender at %s", filepath)

    msg = email.message_from_bytes(envelope.original_content)
    orig = get_orig_message_from_bounce(msg)
    if orig:
        LOG.warning(
            "Original message %s -> %s saved at %s", orig["From"], orig["To"], filepath
        )

    return "250 email to sender accepted"


async def handle(envelope: Envelope, smtp: SMTP) -> str:
    """Return SMTP status"""

    # sanitize mail_from, rcpt_tos
    mail_from = envelope.mail_from.lower().strip().replace(" ", "")
    rcpt_tos = [
        rcpt_to.lower().strip().replace(" ", "") for rcpt_to in envelope.rcpt_tos
    ]
    envelope.mail_from = mail_from
    envelope.rcpt_tos = rcpt_tos

    # unsubscribe request
    if UNSUBSCRIBER and rcpt_tos == [UNSUBSCRIBER]:
        LOG.d("Handle unsubscribe request from %s", mail_from)
        return handle_unsubscribe(envelope)

    # emails sent to sender. Probably bounce emails
    if SENDER and rcpt_tos == [SENDER]:
        LOG.d("Handle email sent to sender from %s", mail_from)
        return handle_sender_email(envelope)

    # Whether it's necessary to apply greylisting
    if greylisting_needed(mail_from, rcpt_tos):
        LOG.warning("Grey listing applied for %s %s", mail_from, rcpt_tos)
        return "421 SL Retry later"

    # result of all deliveries
    # each element is a couple of whether the delivery is successful and the smtp status
    res: [(bool, str)] = []

    for rcpt_to in rcpt_tos:
        msg = email.message_from_bytes(envelope.original_content)

        # Reply case
        # recipient starts with "reply+" or "ra+" (ra=reverse-alias) prefix
        if rcpt_to.startswith("reply+") or rcpt_to.startswith("ra+"):
            LOG.debug(">>> Reply phase %s(%s) -> %s", mail_from, msg["From"], rcpt_to)
            is_delivered, smtp_status = await handle_reply(envelope, smtp, msg, rcpt_to)
            res.append((is_delivered, smtp_status))
        else:  # Forward case
            LOG.debug(
                ">>> Forward phase %s(%s) -> %s",
                mail_from,
                msg["From"],
                rcpt_to,
            )
            for is_delivered, smtp_status in await handle_forward(
                envelope, smtp, msg, rcpt_to
            ):
                res.append((is_delivered, smtp_status))

    for (is_success, smtp_status) in res:
        # Consider all deliveries successful if 1 delivery is successful
        if is_success:
            return smtp_status

    # Failed delivery for all, return the first failure
    return res[0][1]


async def get_spam_score(message: Message) -> float:
    sa_input = to_bytes(message)

    # Spamassassin requires to have an ending linebreak
    if not sa_input.endswith(b"\n"):
        LOG.d("add linebreak to spamassassin input")
        sa_input += b"\n"

    try:
        # wait for at max 300s which is the default spamd timeout-child
        response = await asyncio.wait_for(
            aiospamc.check(sa_input, host=SPAMASSASSIN_HOST), timeout=300
        )
        return response.headers["Spam"].score
    except asyncio.TimeoutError:
        LOG.exception("SpamAssassin timeout")
        # return a negative score so the message is always considered as ham
        return -999
    except Exception:
        LOG.exception("SpamAssassin exception")
        return -999


class MailHandler:
    def __init__(self, lock):
        self.lock = lock

    async def handle_DATA(self, server, session, envelope: Envelope):
        try:
            ret = await self._handle(envelope)
            return ret
        except Exception:
            LOG.exception(
                "email handling fail %s -> %s",
                envelope.mail_from,
                envelope.rcpt_tos,
            )
            return "421 SL Retry later"

    async def _handle(self, envelope: Envelope):
        async with self.lock:
            start = time.time()
            LOG.info(
                "===>> New message, mail from %s, rctp tos %s ",
                envelope.mail_from,
                envelope.rcpt_tos,
            )

            if POSTFIX_SUBMISSION_TLS:
                smtp = SMTP(POSTFIX_SERVER, 587)
                smtp.starttls()
            else:
                smtp = SMTP(POSTFIX_SERVER, POSTFIX_PORT or 25)

            app = new_app()
            with app.app_context():
                ret = await handle(envelope, smtp)
                LOG.info("takes %s seconds <<===", time.time() - start)
                return ret


if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "-p", "--port", help="SMTP port to listen for", type=int, default=20381
    )
    args = parser.parse_args()

    LOG.info("Listen for port %s", args.port)

    if LOAD_PGP_EMAIL_HANDLER:
        LOG.warning("LOAD PGP keys")
        app = create_app()
        with app.app_context():
            load_pgp_public_keys()

    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    lock = asyncio.Lock()
    handler = MailHandler(lock)

    def factory():
        return aiosmtpd.smtp.SMTP(handler, enable_SMTPUTF8=True)

    server = loop.run_until_complete(
        loop.create_server(factory, host="0.0.0.0", port=args.port)
    )

    try:
        loop.run_forever()
    except KeyboardInterrupt:
        pass

    # Close the server
    LOG.info("Close SMTP server")
    server.close()
    loop.run_until_complete(server.wait_closed())
    loop.close()