This How-To will show you how to proxy email traffic so that emails sent to a domain on one server will be proxied over to another server.
To proxy email sent to a domain, you must set that domain in /etc/manualmx.
You can do this by using whmapi1:
How do I add or remove a domain from /etc/manualmx?
An example of this command can be found below:
whmapi1 set_all_proxy_backends domain=example.com mx_host=mailhostexample.com