I just started getting an Outlook error (0x8004210A) trying to receive emails from my main AT&T email address (actually Pacbell.net).
The problem seems to be Outlook thinks there is 1 more new message to receive than are actually available.
I tested this by sending 1 message to myself and then clicking on "Send/Receive all Folders"
I receives the message that I just sent to myself but Outlook continues to wait for another message until the error code is presented.
Outlook is correctly receiving email from my Comcast.net, Hotmail.com, and another Pacbell.net account.
The full error text is
Task 'firstname.lastname@example.org - Receiving' reported error (0x8004210A) : 'The operation timed out waiting for a response from the receiving (POP) server. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).'