Setting Up

SquirrelMail

Configure SquirrelMail's SMTP settings to send through the gateway.

SquirrelMail's outgoing server is set centrally, in its own configuration rather than per user — this is a change for whoever administers the installation.

Run SquirrelMail's conf.pl configuration script (or edit config.php directly), and go to the Server Settings section.
Set the SMTP Server and SMTP Port to the gateway's inbound server settings.
Make sure SMTP authentication is enabled, using each user's own mailbox credentials.
Save the configuration and send a test message from a SquirrelMail account to confirm it routes through the gateway.

This applies to every mailbox on the installation at once — there's no per-user outgoing-server setting in SquirrelMail's own interface.

PHP (mail / PHPMailer / SwiftMailer)

Point a PHP application's outgoing mail at the service.

Ruby on Rails (Action Mailer)

Point a Rails application's Action Mailer SMTP settings at the service.

Python (smtplib)

Point a Python application's smtplib client at the gateway.