[RWilliams]

I think Mail.app does allow quick importing of messages from Entourage, so I will try that, and then copy the messages to the Zimbra IMAP account using Mail. Hopefully that will do the trick.

Edit: Indeed that does work for us. The messages were copied to the Zimbra IMAP account with their original timestamps, so that's one less thing our users will have to worry about. Makes you wonder though why Entourage is incapable of doing the same thing.