We've been having a very hard time getting imapsync to work reliably -- it'll just quit at random spots without any error message. We're not sure if it's our old server or Zimbra.
I did an experiment using the new 5.0 External IMAP feature and except for a few issues it worked a lot better.
After adding an external account the web client gives you an empty folder that you have to right click and say "Get External Mail." Once you've done this the browser will hang for awhile until it gives you the "would you like to cancel this request" window. You can click this and I've noticed the email still downloads in the background, but there is no indication of how long it's going to be before it's done. I suppose that should be a RFE.
The really cool thing, though, is once it is done, all you have to do is move the folders from the external account into your main Zimbra account. This happens instantaneously. Once you've moved the folders you can delete the external account and all is well.
It seems that since Zimbra has the capability of importing external IMAP messages, wouldn't it be great if we could run that process from the command line and load those folders right into the Zimbra account?
Just a thought.