Use these steps to know how do I set up my AT&T email on my Mac device:
From the mail, choose mail > preferences.
Now, choose accounts and then tap to + sign.
After that, choose to add another mail account and then tap to continue.
Complete to add a mail account info like full name as you want to display to others, AT&T email address, and next is to enter the password or secure mail key.
After that, choose to create, and the account will automatically configure and appear in the account list.
Lastly, close the preferences Windows to use the email account smoothly.