Following are the steps you can follow to update or verify your ATT.net email settings.
Go to file > account settings.
Tap the AT&T email account and then tap Change.
Please enter the correct settings for IMAP and POP.
Enter the user's full name next.
Select the remember password checkbox and enter the secure mail key in the password field.
Next, Outlook will test your account settings.
Tap on close, then finish.