Agent Wild Duck

Wild Duck is a distributed IMAP server built with Node.js, MongoDB and Redis. Node.js runs the application, MongoDB is used as the mail store and Redis is used for ephemeral actions like publish/subscribe, locking and caching.

NB! Wild Duck is currently in beta. You should not use it in production.

Goals of the Project

1. Build a scalable and distributed IMAP server that uses clustered database instead of single machine file system as mail store
2. Allow using internationalized email addresses
3. Provide Gmail-like features like pushing sent messages automatically to Sent Mail folder or notifying about messages moved to Junk folder so these could be marked as spam
4. Provide parsed mailbox and message data over HTTP. This should make creating webmail interfaces super easy, no need to parse RFC822 messages to get text content or attachments

Alterntives

Here's a list of Email/IMAP servers that use database for storing email messages

• DBMail
• Archiveopteryx
• ElasticInbox

Supported features

Wild Duck IMAP server supports the following IMAP standards:

• The entire IMAP4rev1 suite with some minor differences from the spec. See below for IMAP Protocol Differences for a complete list
• IDLE – notfies about new and deleted messages and also about flag updates
• CONDSTORE and ENABLE – supports most of the spec, except metadata stuff which is ignored
• STARTTLS
• NAMESPACE – minimal support, just lists the single user namespace with hierarchy separator
• UNSELECT
• UIDPLUS
• SPECIAL-USE
• ID
• MOVE (RFC6851)
• AUTHENTICATE PLAIN and SASL-IR
• APPENDLIMIT (RFC7889) – maximum global allowed message size is advertised in CAPABILITY listing
• UTF8=ACCEPT (RFC6855) – this also means that Wild Duck natively supports unicode email usernames. For example <андрис@уайлддак.орг> is a valid email address that is hosted by a test instance of Wild Duck
• QUOTA (RFC2087) – Quota size is global for an account, using a single quota root. Be aware that quota size does not mean actual byte storage in disk, it is calculated as the sum of the rfc822 sources of stored messages. Actual disk usage is larger as there are database overhead per every message.

Wild Duck more or less passes the ImapTest. Common errors that arise in the test are unknown labels (Wild Duck doesn't send unsolicited FLAGS updates) and NO for STORE (messages deleted in one session can not be updated in another).

FAQ

Does it work?

Yes, it does. You can run the server and get a working IMAP server for mail store, SMTP server for pushing messages to the mail store and HTTP API server to create new users. All handled by Node.js, MongoDB and Redis, no additional dependencies needed. The IMAP server hosting уайлддак.орг uses a MongoDB replica set of 3 hosts.

What are the killer features?

1. Start as many instances as you want. You can start multiple Wild Duck instances in different machines and as long as they share the same MongoDB and Redis settings, users can connect to any instances. This is very different from the traditional IMAP servers where a single user always needs to connect (or be proxied) to the same IMAP server. Wild Duck keeps all required state information in MongoDB, so it does not matter which IMAP instance you use.
2. Super easy to tweak. The entire codebase is pure JavaScript, so there's nothing to compile or anything platform specific. If you need to tweak something then change the code, restart the app and you're ready to go. If it works on one machine then most probably it works in every other machine as well.
3. Works almost on any OS including Windows. At least if you get MongoDB and Redis (Windows fork) running first.
4. Focus on internationalization, ie. supporting email addresses with non-ascii characters
5. +-labels: андрис+ööö@уайлддак.орг is delivered to андрис@уайлддак.орг
6. Access messages both using IMAP and HTTP API. The latter serves parsed data, so no need to fetch RFC822 messages and parse out html, plaintext content or attachments. It is super easy to create a webmail interface on top of this.

Isn't it bad to use a database as a mail store?

Yes, historically it has been considered a bad practice to store emails in a database. And for a good reason. The data model of relational databases like MySQL does not work well with tree like structures (email mime tree) or large blobs (email source).

Notice the word "relational"? In fact document stores like MongoDB work very well with emails. Document store is great for storing tree-like structures and while GridFS is not as good as "real" object storage, it is good enough for storing the raw parts of the message. Additionally there's nothing too GridFS specific, so (at least in theory) it could be replaced with any object store.

You can see an example mail entry here. Lines 184-217 demonstrate a node that has its body missing as it was big enough to be moved to GridStore and not be included with the main entry.

Is the server scalable?

Somewhat yes. Even though on some parts Wild Duck is already fast, there are still some important improvements that need to be done:

1. Optimize FETCH queries to load only partial data for BODY subparts
2. Parse incoming message into the mime tree as a stream. Currently the entire message is buffered in memory before being parsed.
3. CPU usage seems a bit too high, there is probably a ton of profiling to do

How does it work?

Whenever a message is received Wild Duck parses it into a tree-like structure based on the MIME tree and stores this tree to MongoDB. Larger attachments (anything above 50kB) are removed from the tree and stored separately in GridStore. If a message needs to be loaded then Wild Duck fetches the tree structure first, if needed loads attachments from GridStore and then compiles it back into the original RFC822 message. The result should be identical to the original messages unless the original message used unix newlines, these might be partially replaced with windows newlines.

Wild Duck tries to keep minimal state for sessions to be able to distribute sessions between different hosts. Whenever a mailbox is opened the entire message list is loaded as an array of UID values. The first UID in the array element points to the message #1 in IMAP, second one points to message #2 etc.

Actual update data (information about new and deleted messages, flag updates and such) is stored to a journal log and an update beacon is propagated through Redis pub/sub whenever something happens. If a session detects that there have been some changes in the current mailbox and it is possible to notify the user about it (eg. a NOOP call was made), journaled log is loaded from the database and applied to the UID array one action at a time. Once all journaled updates have applied then the result should match the latest state. If it is not possible to notify the user (eg a FETCH call was made), then journal log is not loaded and the user continues to see the old state.

Future considerations

1. Add interoperability with current servers, for example by fetching authentication data from MySQL
2. Maybe allow some kind of message manipulation through plugins? This would allow to turn Wild Duck for example into an encrypted mail server – mail data would be encrypted using users public key before storing it to DB and decrypted with users private key whenever the user logs in and FETCHes or SEARCHes messages. Private key would be protected by users password. For the user the encryption layer would be invisible while guaranteeing that if the user is currently not logged in then there would be no way to read the messages as the private key is locked.

Usage

Assuming you have MongoDB and Redis running somewhere.

Step 1. Get the code from github

$ git clone git://github.com/wildduck-email/wildduck.git
$ cd wildduck

Step 2. Install dependencies

Install dependencies from npm

$ npm install --production

Step 3. Modify config

You can either modify the default config file or alternatively generate an environment related config file that gets merged with the default values. Read about the config module here

Step 4. Run the server

To use the default config file, run the following

npm start

Or if you want to use environment related config file, eg from production.js, run the following

NODE_ENV=production npm start

Step 5. Create an user account

See see below for details about creating new user accounts

HTTP API

Users, mailboxes and messages can be managed with HTTP requests against Wild Duck API

TODO:

1. Expose counters (seen/unseen messages, message count in mailbox etc.)
2. Search messages
3. Expose journal updates through WebSocket or similar

POST /user/create

Creates a new user.

Arguments

• username is the username of the user. This is not an email address but authentication username, use only letters and numbers
• password is the password for the user
• quota (optional) is the maximum storage in bytes allowed for this user. If not set then the default value is used

Example

curl -XPOST "http://localhost:8080/user/create" -H 'content-type: application/json' -d '{
  "username": "testuser",
  "password": "secretpass"
}'

The response for successful operation should look like this:

{
  "success": true,
  "username": "testuser"
}

After you have created an user you can use these credentials to log in to the IMAP server. To be able to receive mail for that user you need to register an email address.

POST /user/address/create

Creates a new email address alias for an existing user. You can use internationalized email addresses like андрис@уайлддак.орг.

Arguments

• username is the username
• address is the email address to use as an alias for this user
• main (either true or false, defaults to false) indicates that this is the default address for that user

First added address becomes main by default

Example

curl -XPOST "http://localhost:8080/user/address/create" -H 'content-type: application/json' -d '{
  "username": "testuser",
  "address": "user@example.com"
}'

The response for successful operation should look like this:

{
  "success": true,
  "username": "testuser",
  "address": "user@example.com"
}

After you have registered a new address then SMTP maildrop server starts accepting mail for it and store the messages to the users mailbox.

POST /user/quota

Updates maximum allowed quota for an user

Arguments

• username is the username of the user to modify
• quota (optional) is the maximum storage in bytes allowed for this user. If not set or zero then the default value is used

Example

curl -XPOST "http://localhost:8080/user/quota" -H 'content-type: application/json' -d '{
  "username": "testuser",
  "quota": 1234567
}'

The response for successful operation should look like this:

{
  "success": true,
  "username": "testuser",
  "previousQuota": 0,
  "quota": 1234567
}

Quota changes apply immediately.

POST /user/quota/reset

Recalculates used storage for an user. Use this when it seems that quota counters for an user do not match with reality.

Arguments

• username is the username of the user to check

Example

curl -XPOST "http://localhost:8080/user/quota/reset" -H 'content-type: application/json' -d '{
  "username": "testuser"
}'

The response for successful operation should look like this:

{
  "success": true,
  "username": "testuser",
  "previousStorageUsed": 1000,
  "storageUsed": 800
}

Be aware though that this method is not atomic and should be done only if quota counters are way off.

POST /user/password

Updates password for an user

Arguments

• username is the username of the user to modify
• password is the new password for the user

Example

curl -XPOST "http://localhost:8080/user/password" -H 'content-type: application/json' -d '{
  "username": "testuser",
  "password": "newpass"
}'

The response for successful operation should look like this:

{
  "success": true,
  "username": "testuser"
}

Password change applies immediately.

GET /user

Returns user information including quota usage and registered addresses

Arguments

• username is the username of the user to modify

Example

curl "http://localhost:8080/user?username=testuser"

The response for successful operation should look like this:

{
  "success": true,
  "username": "testuser",
  "quota": 1234567,
  "storageUsed": 1822,
  "addresses": [
    {
      "id": "58d8fccb645b0deb23d6c37d",
      "address": "user@example.com",
      "main": true,
      "created": "2017-03-27T11:51:39.639Z"
    }
  ]
}

GET /user/mailboxes

Returns all mailbox names for the user

Arguments

• username is the username of the user to modify

Example

curl "http://localhost:8080/user/mailboxes?username=testuser"

The response for successful operation should look like this:

{
  "success": true,
  "username": "testuser",
  "mailboxes": [
    {
      "id": "58d8f2ae240366dfd5d8049c",
      "path": "INBOX",
      "special": "Inbox",
      "messages": 100
    },
    {
      "id": "58d8f2ae240366dfd5d8049d",
      "path": "Sent Mail",
      "special": "Sent",
      "messages": 45
    },
    {
      "id": "58d8f2ae240366dfd5d8049f",
      "path": "Junk",
      "special": "Junk",
      "messages": 10
    },
    {
      "id": "58d8f2ae240366dfd5d8049e",
      "path": "Trash",
      "special": "Trash",
      "messages": 11
    }
  ]
}

GET /mailbox/:id

List messages in a mailbox.

Parameters

• id is the mailbox ID
• size is optional number to limit the length of the messages array (defaults to 20)
• before is an optional paging number (see next in response)
• after is an optional paging number (see prev in response)

Response includes the following fields

• mailbox is an object that lists some metadata about the current mailbox
  • id is the mailbox ID
  • path is the folder path
• next is an URL fragment for retrieving the next page (or false if there are no more pages)
• prev is an URL fragment for retrieving the previous page (or false if it is the first page)
• messages is an array of messages in the mailbox
  • id is the message ID
  • date is the date when this message was received
  • hasAttachments is a boolean that indicates if this messages has attachments or not
  • intro includes the first 256 characters from the message
  • subject is the message title
  • from is the From: field
  • to is the To: field
  • cc is the Cc: field
  • bcc is the Bcc: field

The response for successful listing should look like this:

{
  "success": true,
  "mailbox": {
    "id": "58dbf87fcff690a8c30470c7",
    "path": "INBOX"
  },
  "next": "/mailbox/58dbf87fcff690a8c30470c7?before=34&size=20",
  "prev": false,
  "messages": [
    {
      "id": "58e25243ab71621c3890417e",
      "date": "2017-04-03T13:46:44.226Z",
      "hasAttachments": true,
      "intro": "Welcome to Ryan Finnie's MIME torture test. This message was designed to introduce a couple of the newer features of MIME-aware MUAs, features that have come around since the days of the original MIME torture test. Just to be clear, this message SUPPLEMENT…",
      "subject": "ryan finnie's mime torture test v1.0",
      "from": "ryan finnie <rfinnie@domain.dom>",
      "to": "bob@domain.dom"
    }
  ]
}

GET /message/:id

Retrieves message information

Parameters

• id is the MongoDB _id as a string for a message
• mailbox is optional Mailbox id. Use this to verify that the message is located at this mailbox

Example

curl "http://localhost:8080/message/58d8299c5195c38e77c2daa5"

Response message includes the following fields

• id is the id of the message

• headers is an array that lists all headers of the message. A header is an object:

  • key is the lowercase key of the header
  • value is the header value in unicode (all encoded values are decoded to utf-8)

• date is the receive date (not header Date: field)

• mailbox is the id of the mailbox this messages belongs to

• flags is an array of IMAP flags for this message

• text is the plaintext version of the message (derived from html if not present in message source)

• html is the HTML version of the message (derived from plaintext if not present in message source)

• attachments is an array of attachment objects. Attachments can be shared between messages.

  • id is the id of the attachment
  • fileName is the name of the attachment. Autogenerated from Content-Type if not set in source
  • contentType is the MIME type of the message
  • disposition defines Content-Disposition and is either 'inline', 'attachment' or false
  • transferEncoding defines Content-Transfer-Encoding
  • related is a boolean value that states if the attachment should be hidden (true) or not. Related attachments are usually embedded images
  • sizeKb is the approximate size of the attachment in kilobytes

Embedded images

HTML content has embedded images linked with the following URL structure:

attachment:MESSAGE_ID/ATTACHMENT_ID

For example:

<img src="attachment:aaaaaa/bbbbbb">

To fetch the actual attachment contents for this image, use the following url:

http://localhost:8080/message/aaaaaa/attachment/bbbbbb

Example response

The response for successful operation should look like this:

{
  "success": true,
  "message": {
    "id": "58d8299c5195c38e77c2daa5",
    "mailbox": "58dbf87fcff690a8c30470c7",
    "headers": [
      {
        "key": "delivered-to",
        "value": "andris@addrgw.com"
      }
    ],
    "date": "2017-04-03T10:34:43.007Z",
    "flags": ["\\Seen"],
    "text": "Hello world!",
    "html": "<p>Hello world!</p>",
    "attachments": [
      {
        "id": "58e2254289cccb742fd6c015",
        "fileName": "image.png",
        "contentType": "image/png",
        "disposition": "attachment",
        "transferEncoding": "base64",
        "related": true,
        "sizeKb": 1
      }
    ]
  }
}

GET /message/:mid/attachment/:aid

Retrieves an attachment of the message

Parameters

• mid is the message ID
• aid is the attachment ID

Example

curl "http://localhost:8080/message/58d8299c5195c38e77c2daa5/attachment/58e2254289cccb742fd6c015"

GET /message/:id/raw

Retrieves RFC822 source of the message

Parameters

• id is the MongoDB _id as a string for a message
• mailbox is optional Mailbox id. Use this to verify that the message is located at this mailbox

Example

curl "http://localhost:8080/message/58d8299c5195c38e77c2daa5/raw"

DELETE /message/:id

Deletes a message from a mailbox.

Parameters

• id is the MongoDB _id as a string for a message
• mailbox is an optional Mailbox id. Use this to verify that the message to be deleted is located at this mailbox

Example

curl -XDELETE "http://localhost:8080/message/58d8299c5195c38e77c2daa5"

The response for successful operation should look like this:

{
  "success": true,
  "message":{
    "id": "58d8299c5195c38e77c2daa5"
  }
}

IMAP Protocol Differences

This is a list of known differences from the IMAP specification. Listed differences are either intentional or are bugs that became features.

1. \Recent flags is not implemented and most probably never will be (RFC3501 2.3.2.)
2. RENAME does not touch subfolders which is against the spec (RFC3501 6.3.5. If the name has inferior hierarchical names, then the inferior hierarchical names MUST also be renamed.). Wild Duck stores all folders using flat hierarchy, the "/" separator is fake and only used for listing mailboxes
3. Unsolicited FLAGS responses (RFC3501 7.2.6.) and PERMANENTFLAGS are not sent (except for as part of SELECT and EXAMINE responses). Wild Duck notifies about flag updates only with unsolicited FETCH updates.
4. Wild Duck responds with NO for STORE if matching messages were deleted in another session
5. CHARSET argument for the SEARCH command is ignored (RFC3501 6.4.4.)
6. Metadata arguments for SEARCH MODSEQ are ignored (RFC7162 3.1.5.). You can define <entry-name> and <entry-type-req> values but these are not used for anything
7. SEARCH TEXT and SEARCH BODY both use MongoDB $text index against decoded plaintext version of the message. RFC3501 assumes that it should be a string match either against full message (TEXT) or body section (BODY).
8. What happens when FETCH is called for messages that were deleted in another session? (Not sure, need to check)

Any other differences are most probably real bugs and unintentional.

Future considerations for IMAP extensions

Wild Duck does not plan to be the most feature-rich IMAP client in the world. Most IMAP extensions are useless because there aren't too many clients that are able to benefit from these extensions. There are a few extensions though that would make sense to be added to Wild Duck

1. The IMAP COMPRESS Extension (RFC4978)
2. IMAP4 non-synchronizing literals, LITERAL- (RFC7888). Synchronized literals are needed for APPEND to check mailbox quota, small values could go with the non-synchronizing version.
3. LIST-STATUS (RFC5819)
4. What else? (definitely not NOTIFY nor QRESYNC)

Testing

Create an email account and use your IMAP client to connect to it. To send mail to this account, run the example script:

node examples/push-mail.js username@example.com

This should "deliver" a new message to the INBOX of username@example.com by using the built-in SMTP maildrop interface. If your email client is connected then you should promptly see the new message.

License

Wild Duck Mail Agent is licensed under the European Union Public License 1.1.