As of version 2.3.0, Dovecot provides an SMTP submission service, also known as a Mail Submission Agent (MSA). It is currently implemented as a proxy that acts as a front-end for any MTA, adding the necessary functionality required for a submission service: it 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 8BITMIME and DSN currently require support from the backend/relay MTA.
The most notable feature that the proxy adds is the BURL capability. The main application of that capabilitytogether with IMAP URLAUTHis 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 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 following SMTP capabilities are supported by the Dovecot submission service:
8BITMIME - Only if relay MTA provides support
DSN - Only if relay MTA provides support
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, 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.
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:
- Total number of bytes read from client
- Total number of bytes sent to client
- Number of commands received from client
- Number of replies sent to client
- Session ID of the login session
- ID of the current transaction, if any
- 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.
- 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).
- Maximum number of recipients accepted per connection (default: unlimited).
The Dovecot SMTP submission service directly proxies the mail transaction to the SMTP relay configured with the following settings:
- 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 SSL is used.
- An SMTPS connection (immediate SSL) is used.
- 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.
- Write protocol logs for relay connection to this directory for debugging.
Like IMAP and POP3, the Submission login service supports proxying to multiple backend servers. The proxy configuration wiki page for POP3 and IMAP applies automatically to Submission as well.