We're just testing out Zimbra in hopes to roll it out soon. It was quite a job to get the ZCO working for one user, but it seems to mostly work.
Problem...sending out to email addresses NOT hosted on the zimbra server fails with:
The following recipient(s) cannot be reached:
'firstname.lastname@example.org' on 31/05/2008 01:12
554 5.7.1 <email@example.com>: Relay access denied