FYI.
i was using CWP to provide simple web hosting for a couple of WP websites.
All email/DNS etc was dealt with elsewhere.
in the end i added an SMTP plug in for Worpress that allowed me to set an external SMTP relay for all wordpress email.
this circumvented the local email/smtp service.
C.
Hi, i have same config as yours... i did this
phpmyadmin - postfix - domain
find domain you dont want to recieve email, change "active" from 1 to 0...
You can still open webmail for this domain, but even if you send email for yourseft, email will properly routed to server which mx record pointed.
ps. i hope you understand, as my english is google translated