Plesk servers typically send outgoing mail through Postfix — routing it through the gateway is a server-level change, made by whoever administers the Plesk installation.
- In the Plesk admin panel, go to Tools & Settings → Mail Server Settings.
- Under Relaying, configure outgoing mail to relay through a smart host, pointed at the gateway's inbound server, with authentication credentials for your gateway account.
- Apply the change and send a test message from a mailbox on the server to confirm it now routes through the gateway.
See Postfix for the underlying configuration this builds
on, if your Plesk setup needs main.cf/sasl_passwd edited directly rather than
through the Plesk interface.
As with cPanel, this affects every mailbox on the server at once — check whether
your Plesk version supports relay settings scoped to a single domain if you only
want to route some senders through the gateway.