I have an interesting one where by an Outlook 2007 client is downloading email again and again. The client connects to outlook.com via pop and it is set to leave the message on the server.
The reason its set to pop is because multiple people connect to this single mailbox to pull the mail down so I didn’t want to configure it to imap in case someone deleted the email which would them remove it from the server and the other mailboxes.
It doesn’t do it on the 2010 outlook clients though???