This is a server problem for POP accounts. You might try using web login to see if some messages are corrupted.

I didn't see your reply. I apologise. I don't know what web login is!

Update: I've seen a few more posts that seem similar to yours, and I figured their problem was that the email host had just converted to Exchange as an access. If so, you may wish to try one of these addons, OWL or EXQUILLA, to use Exchange for your account. But first, I suggest verifying that this is the case.

Thank you. Help is highly appreciated.