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Submission Server
=================

As of version 2.3.0, Dovecot provides an SMTP submission service, also known as
a Mail Submission Agent (MSA) [https://tools.ietf.org/html/rfc6409]. It is
currently implemented as a proxy that acts as a front-end for any <MTA.txt>,
adding the necessary functionality required for a submission service: it adds
the required AUTH [https://tools.ietf.org/html/rfc4954] support, avoiding the
need to configure the MTA for SASL authentication. More SMTP capabilities like
CHUNKING [https://tools.ietf.org/html/rfc3030] and SIZE
[https://tools.ietf.org/html/rfc1870] are supported, without requiring the
backend MTA supporting these extensions. Other capabilities like 8BITMIME
[https://tools.ietf.org/html/rfc6152] and DSN
[https://tools.ietf.org/html/rfc3461] currently require support from the
backend/relay MTA.

The most notable feature that the proxy adds is the BURL capability
[https://tools.ietf.org/html/rfc4468]. The main application of that
capabilitytogether with <IMAP> [IMAPServer.txt] URLAUTH
[https://tools.ietf.org/html/rfc4467]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. Few clients
currently support the BURL capability, but once it becomes available on the
server side, client developers will at least have some incentive to provide
support for this feature.

*NOTE:* Currently, 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 that could be implemented are
adding <Sieve> [Pigeonhole.txt] 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.

Features
--------

The following SMTP capabilities are supported by the Dovecot submission
service:

 * 8BITMIME [https://tools.ietf.org/html/rfc6152] - Only if relay MTA provides
   support
 * AUTH [https://tools.ietf.org/html/rfc4954]
 * BURL [https://tools.ietf.org/html/rfc4468]
 * CHUNKING [https://tools.ietf.org/html/rfc3030]
 * DSN [https://tools.ietf.org/html/rfc3461] - Only if relay MTA provides
   support
 * ENHANCEDSTATUSCODES [https://tools.ietf.org/html/rfc2034]
 * PIPELINING [https://tools.ietf.org/html/rfc2920]
 * SIZE [https://tools.ietf.org/html/rfc1870]
 * STARTTLS [https://tools.ietf.org/html/rfc3207]
 * VRFY [https://tools.ietf.org/html/rfc5321]
 * XCLIENT [http://www.postfix.org/XCLIENT_README.html]

Configuration
-------------

Submission Service
------------------

Just add 'submission' to the 'protocols=' setting and configure the relay MTA
server. The submission service is a login service, just like IMAP, POP3 and
<ManageSieve> [Pigeonhole.ManageSieve.txt], so clients are required to
authenticate. The same <authentication configuration> [Authentication.txt] 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.

The following settings apply to the Submission service:

submission_logout_format = in=%i out=%o:
  The SMTP Submission logout format string. The following variable
  substitutions are supported:
  %i :
    Total number of bytes read from client

  %o :
    Total number of bytes sent to client

  %{command_count} :
    Number of commands received from client

  %{reply_count} :
    Number of replies sent to client

  %{session} :
    Session ID of the login session

  %{transaction_id} :
    ID of the current transaction, if any

hostname :
  The host name reported by the SMTP service (and other parts of Dovecot), for
  example to the client in the initial greeting and to the relay server in the
  HELO/EHLO command. Default is the system's real hostname@domain.

submission_max_mail_size :
  The maximum size of messages accepted for relay. This announced in the SMTP
  SIZE capability. If not configured, this is either determined from the relay
  server or left unlimited if no limit is known (the relay MTA will reply with
  error if some unknown limit exists there, which is duly passed to Dovecot's
  client).

submission_max_recipients :
  Maximum number of recipients accepted per connection (default: unlimited).

Relay MTA
---------

The Dovecot SMTP submission service directly proxies the mail transaction to
the SMTP relay configured with the following settings:

submission_relay_host :
  Host name for the relay server (required).

submission_relay_port = 25 :
  Port for the relay server.

submission_relay_trusted = no :
  Is the relay server trusted? This determines whether we try to send
  (Postfix-specific) XCLIENT data to the relay server.

submission_relay_user = :
  User name for authentication to the relay MTA if authentication is required
  there (authorization ID).

submission_relay_master_user = :
  Master user name for authentication to the relay MTA if authentication is
  required there (authentication ID).

submission_relay_password = :
  Password for authentication to the relay MTA if authentication is required
  there.

submission_relay_ssl = no :
  Indicates whether TLS is used for the connection to the relay server. The
  following values are defined for this setting:
  no :
    No SSL is used.

  smtps :
    An SMTPS connection (immediate SSL) is used.

  starttls :
    The STARTTLS command is used to establish the TLS layer.

submission_relay_ssl_verify = yes :
  Configures whether the TLS certificate of the relay server is to be verified.

submission_relay_rawlog_dir :
  Write protocol logs for relay connection to this directory for debugging.

See <Debugging.Rawlog.txt> for more details on rawlogs.

Login Proxy
-----------

Like IMAP and POP3, the Submission login service supports proxying to multiple
backend Dovecot servers. The <proxy configuration wiki page>
[PasswordDatabase.ExtraFields.Proxy.txt] for POP3 and IMAP applies
automatically to Submission as well.

*IMPORTANT*: Please note that the login proxy described here is configured
between two Dovecot servers (e.g. director frontend and mail storage backend).
This is not the way to configure the relay connection between the Dovecot
submission service and the MTA! That is configured using the relay settings
described in the <previous section> [Submission.txt]. If you get this wrong,
things will seem to work (at least to some extent), but the service provided by
Dovecot will be effectively bypassed.

Client Support
--------------

As of April 2019 there aren't many clients that support the BURL extension.
+-------------+-----------+-------------------------------------------------------------------------------------------------------+
| *Client*    | *Supports | *Notes*                                                                                               |
|             | BURL?*    |                                                                                                       |
+-------------+-----------+-------------------------------------------------------------------------------------------------------+
| Trojita     | Yes       | Mentioned on the dovecot discussion list                                                              |
|             |           | [http://dovecot.2317879.n4.nabble.com/New-Dovecot-service-SMTP-Submission-RFC6409-tp62117p62122.html] |
+-------------+-----------+-------------------------------------------------------------------------------------------------------+
| Thunderbird | No        | Vote on bugzilla [https://bugzilla.mozilla.org/page.cgi?id=voting/user.html&bug_id=421779]            |
+-------------+-----------+-------------------------------------------------------------------------------------------------------+
| KDE Akonadi | No        | KDE bugzilla [https://bugs.kde.org/show_bug.cgi?id=408732]                                            |
| / KMail     |           |                                                                                                       |
+-------------+-----------+-------------------------------------------------------------------------------------------------------+

(This file was created from the wiki on 2019-06-19 12:42)