Problem with MTA

Good evening.

For the moment I have a really serious problem. Today we made the upgrade from GW14.2.2 to GW18.1. So far, so good. Now I have following situation: Internal eMails are working. From outside I see a large number of messages in gwia. But there is nearly no traffic between my servers.
OS: SLES11 Sp4, OES2015 (all actual patches are installed). In my Server-LAN I have one machine (Primary-Domain,MTA,PO)and another in the DMZ (Secondary-Domain, MTA, GWIA,Webacc).

The only changes we made after upgrade is to change the prefedered Internet-domain in System -> Internetsettings. Her I opened a SR with critical. The support searches several hours for a solution. For today, we gave up here :(.
The change of the intern-domain was rolled back.

Any ideas to fix it?

Thanks a lot in advance for your answers.

Regard

Dirk Emmermacher