Works for me. There was a problem matching your description with Mac OS 10.5.7 and Zimbra 5.0.16 (possibly earlier versions of Zimbra as well) but it was solved with Zimbra 5.0.18 (5.0.17 was never released). And it continues to work with Mac OS 10.5.8.
In normal operation, you should see your responses in your Sent folder of Mac Mail.
I'd suggest you ask your campus what version of Zimbra they've got running.
See also
http://www.zimbra.com/forums/users/3...vitations.html
Can't help you with the 10.6 issue as my copy hasn't arrived yet.