This is because VPOP3 version 5 and later support BATV. BATV helps to eliminate 'backscatter spam', where someone else sends spam pretending to be from you, and then you receive all the bounce messages from those messages. BATV encodes the sender address in a special way so that VPOP3 can detect that a bounce message is in response to a message that you actually sent.

Usually this is absolutely fine, but if your ISP only allows sending from or receiving to specific email addresses, and does not understand the BATV system, it may cause problems.

To turn off BATV support in VPOP3 v6.7 and later, go to Settings → Message Authentication → BATV, and uncheck the Enable BATV Support box, and press Submit

To turn off BATV support in VPOP3 v5-v6.7, go to Settings → Misc Settings → BATV, and uncheck the Enable BATV Support box, and press Submit