I use Groupwise at work, and other accounts for everything else, and I have them all in Mail.app so I can go through all my new mail quickly.

Mail.app cannot do Groupwise calendaring and appointments, so I also use the Mac Groupwise client occasionally.

Imagine my horror one day, when looking in the Groupwise account's Inbox in Mail.app and finding that it was completely empty. Several thousand messages - disappeared!

It turned out that in the Groupwise client, I'd accidentally moved the Mailbox (Groupwise's inbox) into another folder. The Mac Groupwise client is quite "twitchy": it is very easy to move this folder without even noticing you've done it. It only takes a very small motion and less than a second to send it whirling into an unknown folder somewhere else in the tree. It takes a lot longer to FIND the Mailbox folder you've moved, since it can end up almost anywhere. But once you move it back out, your mail will show up again in Mail.app and other mail clients.

Be careful with that mouse!