Here are the steps you can take to verify or update your ATT.net email settings.
Go to file > account settings.
You can change your AT&T email account by tapping on it.
Make sure you have proper POP and IMAP settings.
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 the account settings.
Tap on close, then finish.