New Dovecot service: SMTP Submission (RFC6409)

[Stephan Bosch]

Hi,

As some of you know, I started implementing the SMTP submission proxy a
few years ago. It acts as a front-end for any MTA, adding the necessary
functionality for an SMTP submission service, also known as a Mail
Submission Agent (MSA) (https://tools.ietf.org/html/rfc6409). The main
reason I created this, back then, was implementing the BURL capability
(https://tools.ietf.org/html/rfc4468). The main application of that
capability -- together with IMAP URLAUTH -- is avoiding a duplicate
upload of submitted e-mail messages; normally the message is both sent
through SMTP and uploaded to the "Sent" folder through IMAP. Using BURL,
the client can first upload the message to IMAP and then use BURL to
make the SMTP server fetch the message from IMAP for submission, thereby
avoiding a second upload. Apart from BURL, the submission proxy service 
also adds the required AUTH support, avoiding the need to configure the
MTA for SASL authentication. More SMTP capabilities like CHUNKING and
SIZE are supported, without requiring the backend MTA supporting these
extensions. Other capabilities like DSN currently require support from
the backend/relay MTA.

At this point, the submission proxy is still pretty basic. However, it
will provide a basis for adding all kinds of functionality in the (not
so distant) future. For the first time, it will be possible to act upon
message submission, rather than only message retrieval; e.g. plugins can
be devised that process outgoing messages somehow. Examples of the
things we could do are adding Sieve filtering support for outgoing
messages, or implicitly storing submitted messages to the Sent folder.
Once a plugin API is devised, you can create your own plugins.

The reason I send this message now, is that this code is finally merged
into the Dovecot master repository. This means that it is part of the
upcoming 2.3 release. Now that it is merged, you can install and test it
from Github if you like. Feedback is of course appreciated. The
documentation is still pretty sparse, but there is currently not much to
configure. Just add "submission" to the protocols and configure the
relay MTA server. The configuration is currently only documented in the
example configuration in doc/example-config/conf.d/20-submission.conf.
The submission service is a login service, just like IMAP, POP3 and
ManageSieve, so clients are required to authenticate. The same
authentication configuration will also apply to submission, unless
you're doing protocol-specific things, in which case you may need to
amend your configuration for the new protocol. BURL support requires a
working IMAP URLAUTH implementation.

I've updated the automated Xi Debian package builder to create an
additional dovecot-submissiond package. So, if you're using the Xi
packages, you only need to install that package and configure the relay MTA.

Regards,

Stephan.