Are you using Outlook, Window Live Mail or something else? What is your email domain, last half of your email @example.com
Can we setup remote session to further troubleshoot?
I had someone else with att.net through Yahoo also have issues with POP3, it seems to be an issue on their side.
The fix was switching to IMAP.
Checking in to see if you still need help with this issue, if so please reply at your convenience.