Ok, so you have a plesk server and your hosting a site for a client who wants to use thier own mail server. So you go ahead and change the mx record to point to thier mail server and email the client back to let him know everything is completed. A day later you realize that your client isn’t receiving emails from you. In fact he’s not receiving emails from his friend Jim (also a client of yours hosted on the same physical server). But he is receiving email from just about everyone else in the world. What the heck is going on you ask yourself!
Well, normally when someone sends an email, the mx record is looked up and followed accordingly. However, plesk uses qmail as it’s MTA (Mail Transit/transport Agent). Since Jim and you are using the same server that host your clients site, qmail assumes it is handling the email for that domain as well. It’s an easy fix though:
Just open in vi or pico or whatever your using these days the following two files and comment out the clients domain who is using an external mail server.
Then restart qmail so it reloads the new configuration.
/sbin/service qmail restart