postfix rejects messages

[Stefan]

Hello List!

This question is a little bit special :-)

The kolab2-server is my internal mail-server and its relay is another machine 
in our DMZ. 
Now I want to send and receive mails with my fine SE P900 (handy). For 
security reasons, I want to do this via our DMZ-Mail-Server. On the P900 I 
use the same mail address as with kontact, because I don't want to confuse 
the receipient with to many mail addresses. I get this working so far, but 
when I try to send a mail to an internal (kolab) mail address, kolab rejects 
the mail coming from the DMZ-Mail-Server (Invalid Sender). 
Certainly, this behavior of kolab is no bug, but is there a way to get kolab 
accepting this mails??

TIA
Stefan