100% Dovecot MTA Replacement Setup

[David Favor]

Working out upgrade of old 1.x + 2.x Dovecot installs to
latest 2.3.2.1 out of Ubuntu Bionic repositories.
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https://wiki.dovecot.org/Services suggests Dovecot can
be used for full mail infrastructure, avoiding the
complexity of exim4 or other MTA setup/management.

Let me know if I understand this correctly.

1) https://wiki.dovecot.org/Submission can be used to
    listen on port 25 + port 587 (with auth).

2) https://wiki.dovecot.org/Pigeonhole can be used for
    filtering, including forwarding email to other machines.

3) https://wiki.dovecot.org/Pigeonhole/ManageSieve/Configuration
    can be used to forward off machine email, for example flowing
    through a relay service like MailGun, using a simple script
    or service like ESMTPD.

4) https://wiki.dovecot.org/LMTP can be used to deposit messages
    into filesystem as Maildir or dbox or mdbox backing stores.