Zimbra's a pretty neat client! I daresay it looks like it's almost a gmail killer, and definitely a gmail killer in that one can use it without outsourcing mail hosting to a single company out of the country.
However, I'm trying to figure out how well Zimbra would handle a user who participates heavily in mailing lists (remote ones, not Zimbra-hosted).
I've managed to boil it down to these questions:
1) Does Zimbra use Maildir to store messages in the back end?
2) Can procmail be used at all? (I have a recipe that automatically creates new folders for each mailing list based on incoming messages' list-id). I suppose it would be better to use Zimbra's message filter rule facility, but it doesn't appear to support list-id.
3) Does Zimbra now do proper threading based on message-id? I read that back in 2005 Zimbra used the subject line to do it, which isn't very nice.
4) Does Zimbra store messages via Maildir, mbox, or its own database back end?
5) Has anyone worked on gpg support (OK, maybe this one is asking a bit much)?