app/templates/emails/transactional/reply-must-use-personal-email.txt

{% extends "base.txt.jinja2" %}

{% block content %}
We have recorded an attempt to send an email from your alias {{ alias.email }} using {{ sender }}.

Please note that sending from this alias only works from alias's owning mailboxes, that are
    {{ mailbox_emails|join(", ") }}

This is to protect against "email spoofing", i.e. someone else sending emails from your mailbox.
If {{ sender }} can indeed send emails from your alias, you can add it to the allow list on {{authorize_address_link}}
{% endblock %}