Just to be clear, you are using pop in Windows Mail, not a web mail, correct?

First, if you are using SSL settings, they need to be disabled in the email client and enabled in avast.

Configure avast mail shield correctly (with the correct email account information).

Finally, delete your firewall rules related to both, avast and your email client. The should be re-created when the respective programs try to connect.

If, as oppose to what I understood from your post, you are using webmail, then please give more details as the problem might be in the web shield instead of the mail shield.

Please report back.