Cpanel is trying to deliver mail to the local server as the domain you are trying to send to is more than likely hosted on the same server.

When setting up the web domain Cpanel creates localdomains hence it tries to deliver to itself.

To fix simply edit the localdomains file and remove the domain in question:

vim /etc/localdomains

Delete the domain and write/quit.

Written by Matt Cooper
Hi, I'm Matt Cooper. I started this blog to pretty much act as a brain dump area for things I learn from day to day. You can contact me at: matt@linuxtutorial.co.uk.