Roundcube's outgoing server is usually set by whoever administers the Roundcube installation, in its global configuration — but it can also be set per identity if your installation allows it.
Per-mailbox (if enabled)
- In Roundcube, go to Settings → Identities, and select the identity you send from.
- If your installation exposes SMTP settings here, enter the gateway's inbound server settings — server, port, and your account email as the username.
- Save, then send a test message.
Server-wide (for a Roundcube administrator)
If per-identity SMTP settings aren't available, the outgoing server is set in
Roundcube's own config.inc.php (the smtp_host, smtp_port, smtp_user, and
smtp_pass settings), pointing at the
gateway's inbound server settings instead of your
mail server's own SMTP service. This applies to every mailbox on that Roundcube
installation.
Which option applies to you depends entirely on how your Roundcube instance is
configured — check with whoever administers it if you're not sure which one you
can use.