Like most webmail installations, Horde's outgoing mail settings are usually
configured centrally by whoever administers it, in the IMP module's mail server
configuration (backends.php), rather than by individual users.
- In Horde's mail server configuration, find the SMTP transport settings for the backend you want to route through the gateway.
- Set the host and port to the gateway's inbound server settings.
- Set authentication to use each user's own mailbox credentials, matching how your installation is already set up to authenticate outgoing mail.
- Save the configuration and send a test message from a Horde mailbox to confirm it routes through the gateway.
If you're a Horde user rather than its administrator, this change needs to be made
by whoever manages your installation — there's no per-user SMTP setting in the
standard Horde webmail interface.