I don't think mta service is not running because of DNS issue. Other email service like sendmail might have aquired the port 25, hence Zimbra postfix failing to start. What do you see in /var/log/zimbra.log when you run following command:
su - zimbra
zmmmtactl start
For the DNS issue:
You are trying to configure a splitDNS setup. Where Zimbra server is behind a firewall and needs to resolve its address in internal network. Following page describes the situation and second page gives direction to configure DNS on a SLES system:
Split DNS - Zimbra :: Wiki SUSE Linux Enterpise Server 9 NAT HOWTO - Zimbra :: Wiki