corrupted e-mail?
Here is what worked for me after Verizon wasted my time checking Outlook settings that never had changed. After that I noticed (via the web browser) that I had about 50 messages, but Outlook was only downloading about 15 before blowing up with the error. So I deleted that last downloadable e-mail and the one before and after it. After that the problem cleared up.
Your ISP will make you go to your e-mail via a browser and send yourself a message. If it gets received by the server (as seen via browser), then they will say the server is fine and it is your outlook. In my case it seems to be a corrupted e-mail.
Receiving' reported error (0x800CCC90) : 'Your incoming (POP3) e-mail server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP)