Re ;
External mail feature is not implemented in stable.. Read http://forum.i-mscp.net/Thread…eature-New-implementation carefully and you will surely understand how to solve your problem.
In order you have to:
1. Replace default MX entry set by imscp by your external MX (in your bind zone file)
2. Remove the domain entry from the postfix virtual_mailbox_domains map
3. Add the domain into to the Postfix relay_domains map (see the postfix doc about that)
restart all.
The main problem is that the change are volatile... I suggest to upgrade to master and use new external mail feature.