Follow these steps:
1. After the IMAP protocol got introduced, many email service providers including ATT opt for IMAP protocols leaving POP.
2. The users who are still attached around POP instead of POP and having an issue. Then they all are suggested to switch to IMAP.
So these all steps that one should perform to solve AT&T Email not Working with Outlook. If you all are satisfied with the solution provided above, then I will share other posts related to the AT&T mail down the issue. The AT&T customer service team 24*7 ready to guide.