Archived - Mac OS X Server v10.5, 10.6: How to specify a port for SMTP relay
The Mail Service in Mac OS X Server can be configured to forward all outgoing mail to an external mail server. By default, when you set up forwarding, the Mail service forwards outgoing messages to port 25 on that server. Some service providers may block port 25, so you can specify a different port in Server Admin.
To forward to a port other than 25, simply append a colon and the port number to the host or IP address of the target server when configuring SMTP relay in Server Admin.
For example, to forward outgoing mail to port 2525 on relay.example.com, enter relay.apple.com:2525 in the "Relay outgoing mail through host" field.