I've been experiencing a strange but quite serious bug in Outlook 2003 which only started after installing the Zimbra Connector.
Occasionally when I spend a long time writing a message, Outlook reverts to an old saved draft after clicking "Send" and before the message is actually sent. This results in an unfinished (sometimes embarrassingly so) email being sent out instead of the version that existed at the time of sending.
This is hard to reproduce, since it only happens perhaps 1 out of every 2,000 emails sent. My best guess is that there's some kind of narrow timing window, possibly relating to the spell check that it runs before sending. Also (and this could be a red herring), I'm pretty sure it only happens after I've changed the email format from Plain Text to HTML.