Often this is because there is 'Read Receipt' stuck in the Outlook out queue, but these do not show in the Outlook Outbox view.

We found this article helpful: http://www.howto-outlook.com/howto/deletereadreceipt.html

If you need help with following this, you can call us, but it will be a chargeable support call as this problem has nothing to do with VPOP3 at all.