I tried Zarafa (great system) with z-push but ran into some issues. I had some exhaustive irc sessions with one of the developers and it seems that there are some bugs in the Mfe software from Nokia. There are (at least) three issues:

1) on an initial sync, the N900 creates a second connection during the sync where it tries to sync the exact same items as the first connection. So you end up with a lot of duplicates. An ugly patch in z-push can prevent that but has to be disabled after initial sync.

2) the N900 doesn't follow the Activesync specifications with timezones. Full day events are not synced correct. An ugly patch in z-push can help but not when you travel across timezones.

3) Some pictures from N900 contacts aren't synced so they are empty on Zarafa. On the next sync they are removed some N900 too. The N900 doesn't see the pictures when syncing. I checked that in the N900 synclog.

These issues are reported to Vitaly of Nokia.

So unfortunaty the combination of Zarafa with N900 doesn't work well in firmware 1.0 and 1.1. This is not a problem of Zarafa but the Mfe implementation of the N900. The Zarafa developer was really helpfull and spent a couple of hours debugging the syncing.