auth/auth_proxy plugin
This plugin allows you to authenticate users by domain to remote SMTP servers and proxy the result back to authenticate the client.
For this to work - the AUTH username must be in user@domain.com format regardless as to whether the remote SMTP server requires it in this format. The domain part of the username is used to look-up which SMTP servers should be used to authenticate users for that domain. When sending the AUTH credentials to the remote server, this plugin will try and send the full username e.g. user@domain.com first and if this fails it will then strip the @domain.com part and just send the unqualified username.
Due to the way this plugin works - it can only support PLAIN and LOGIN authentication methods and for this reason it requires that STARTTLS be used via the tls plugin before it will advertise AUTH capabilities by the EHLO command. When connecting to the remote SMTP systems it will always attempt STARTTLS if it is offered, but it does not require it, so caution should be exercised.
Configuration
Configuration is stored in config/auth_proxy.ini and uses the INI
style formatting.
The configuration of this plugin is simple:
[domains]
domain.com = server1.domain.com:587 server2.domain.com
Where domain.com is the domain-part of the username equals a list of hosts
that should be consulted in host:port format. The
If more than host is specified, then subsequent hosts will only be tested if there is some sort of error e.g. timeout, connection or protocol error.