This seems to be almost 2 months old. Did you find a solution to your issue in the meantime?
Lately there was an update to Kolab KE14 on UCS. Perhaps you can try to update your installation and see if that resolves your issue.
If this is not making your mail flow, then perhaps you can add a few more details:
Do you get any error messages anywhere? In log files? On the screen?
Is the mail bouncing and returned to UserA (since it is not arriving at the UserB mailbox), or is it just disappearing?
Am I interpreting it correctly, that you your server is a member of a domain, but is sending mail via a proxy? What does that mean for the FQDN? Can you clarify?
You can find documentation for Kolab at: docs.kolab.org/ .
Hope it's helpful