I had a customer with a pop3 account switching to IMAP.

So he created a new imap account within his Oultook profile, then he moved his mail from the pop3 account into to imap account so that his mail would be uploaded to the mailserver.

But somehow most of the messages (a lot of them 20mb+)where uploaded multiple times consuming the harddiskspace of the server.
I am cleaning up the mess now, and it's a pitty ít's just before we were moving to a new server.