Email delivery problem

[crankshaft25]

Hello,
We've recently setup zimbra NE 5.0.18 on Ubuntu 8.04LTS. Receiving email and sending seem to be working most of the time. I did have a relay problem but solved that.
Now the problem seems to be with some of the messages I see in the logs and from users reporting sending problems.
Our zimbra is behind a firewall/router and has both and internal IP address and external IP address served by two seperate NIC's. The external NIC is on our DMZ with an IP address of xxx.xxx.xxx.101 the IP address of our gateway is xxx.xxx.xxx.98 (on the same subnet). Now what I am seeing in the logs is as follows
5.7.1 Client host rejected: cannot find your reverse hostname,

[xxx.xxx.xxx.98] (in reply to RCPT TO command)
It looks like zimbra is sending out email using the Ip of the gateway. I don't remember this being a problem on our old basic postfix install before the migration.
I don't have split DNS setup on the zimbra server if that's any help. I didn't know it was required as our old server didn't need it.