If the problem always occurs after you have downloaded the same number of messages, then it is most likely caused by a "corrupted" email message. All mail clients have a problem: they sometimes "think" that perfectly valid email messages are corrupt. Different mail clients have this problem with different email messages.

The "bad" message is either going to be the last message you downloaded, or the first message you didn't download.

There are three ways around this issue. Any one of these should fix the problem:

  1. Open your mailbox in webmail. Delete the last message you receive, as well as the first message after that (the first message you are unable to download).  
  2. Use a different mail client to connect to the server using IMAP instead of POP. Delete the last message you receive, as well as the first message after that (the first message you are unable to download).
  3. Use a different mail client to download these messages in POP, setting it to NOT leave a copy of them on the mail server. Once the "problem" message is no longer on the server, your normal mail client shouldn't have any problems. The only down side to this method is that you will have those messages stored in a different mail client.

If it always errors out on the first message (that is, before you download any mail), then deleting the least recent message should fix the problem.