To know how do I set up my AT&T email on my Mac device, then check out the steps as mentioned in the guide:
From mail, choose mail > preferences.
Select account and choose the + sign.
Tap to add another mail account and click to continue.
Fill up the add a mail account info:
AT&T email address
Password: enter the password or secure mail key
Choose to create, and the account will automatically configure and appear in the accounts list.
Lastly, select the preferences window to begin using an email account.
Use these steps to set up your email for the first time or on a new computer. From Mail, select Mail > Preferences. Choose Accounts and then select the + (plus sign). Select Add Other Mail Account and then Continue. Complete Add a Mail Account info: Full name as you want it to appear to others AT&T email address (YourName@att.net) Password – enter your password or secure mail key Select Create. The account will automatically configure and appear in the Accounts list. Close the Accounts window to begin using your email account.