Your system being connected with stable internet connection.
Take a backup of your ATT email account.
Open Outlook account in safe mode.
Be ready with correct login details like email address, username and password.
The AT&T Outlook Email Settings should be held.
Above mentioned are the basic requirements for configure AT&T Email in MS Outlook.
See Also this post:- http://datarecovo.populr.me/how-to-set-up-att-email-on-ms-outlook
Set up Outlook 2016 for AT&T email
Use these steps to set up your email for the first time or on a new computer.
In Outlook, select File > Add Account.
Choose Manual setup or additional server types.
Select Pop or IMAP.
Complete your User Information:
Enter your name as you want it to display to others.
Enter your full AT&T email address.
Complete the Server Information:
Account Type - Select IMAP or POP3.
Incoming mail server - Enter imap.mail.att.net (IMAP) or inbound.att.net (POP3).
Outgoing mail server - Enter smtp.mail.att.net (IMAP) or outbound.att.net (POP3).
Complete the Logon Information:
User Name - Enter your full email address.
Password - Enter or paste your secure mail key.
Check Remember password and Require logon using Secure Password Authentication.
Select More Settings.
Complete Outgoing Server info:
Check My outgoing server (SMTP) requires authentication.
Select Use same settings as my incoming mail server.
Complete Advanced info:
Incoming server - Enter 993 (IMAP) or 995 (POP3).
Use the following type of encrypted connection - Choose SSL.
Outgoing server - Enter 465 (IMAP and POP3).
Use the following type of encrypted connection - Choose SSL.
Select OK and then Next. Outlook will test your account info. If everything passes, select Finish.
Regards, Bruce