we are about to start looking at mail archiving solutions and instead of re-inventing the wheel I thought I would ask fellow Zimbra users what you are using ? MailArchiva is probably at the top of my list at the moment though that can change. I did take a cursory look at Enkive but that appears to new at present and lacks quite a lot of functionality eg. data guardians. Look forward to your replies.
I've used both, the 'bcc to archive user' technique (described here) and MailArchiva, and the latter is damn good at quickly retrieving mails. Although we did face intermittent problems while initially setting it up. Can't remember what it was thought..