The Zarafa Contrib is still being developed. There are a few bugs, which you can check out via the bug tracker.
Zarafa has several ways to deal with users. Currently, and I point out this is still up for discussion, Darrell has implemented the database option (db). With this option, Zarafa maintains its own list of users and does not rely on the SME users. A problem with this approach is that email received for that user will be rejected unless there is an identical SME user. This requires separate entries for each user, first via the SME control panel and secondly via the zarafa-admin command line. The order in which the users are created makes a difference, with the SME user having to be created first.
MJ