Need a solution

[joximu]

He also should delete anything referring the external maildomain from his postfix files.

I just made a test (because of ourt hijacking problem):
I deleted the domain gmx.net on my test ispcp-server but let the catchall for gmx.net in the postfix configuration. bind restart. postfix restart -> still the mails send from the system to gmx are going where I configured my hijacking account...

So, for postfix it's more than only the dns entries...