Migration issue - mail server thinks its to keep email when it should forward it

Hello all...

So i am migrating some domains from old ispcp to ispconfig... so to make things smooth i add all domains and email boxes from the old infrastructure to ispconfg. All works ok, however when i have a client that has 2 domains one on old infrastructure one on ispconfig, when they send mail from one domain1.com to domain2.com it stays on ispconfig server rather than be delivered to old infrastructure.

IE;
domain 1.com is ACTIVE on old infrastructure and is in ispconfig for the migration later on, but has the mail domain as NOT active
domain2.com is ACTIVE on ispconfig

So when someone with a domain2.com email sends email to an email at domain1.com, it stays on ispconfig server rather than going to old infrastructure!

Now like i said domain1.com is in ispconfig, but marked as INACTIVE, but yet emails still get delivered locally...
So the clients in this situation, about a dozen or so, have to use webmail on ispconfig to get their emails that should be going to old infrastructure.
I could simply manually edit the transport file on ispconfig server, but that would get overwritten when changes are made...
Any ideas how to remedy this?

Thanks..