Well the problems I was having above were because I didn't have any DNS set up correctly. Once I reloaded it with BIND and configured BIND properly, the IM worked just fine.
You might want to check to make sure that IM is enabled in the class of service for both users.
Also, can you send e-mail back and forth between these two users? Are you maintaining everything to one network - does Zimbra have to go outside of your network for DNS resolution and routing?
- Jesse | 
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