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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.