A subaccount is a Member ID email address set up under the account owner’s primary Member ID. Only the account owner can create and manage subaccounts. For example, the account owner would have a primary Member ID email and their spouse or children would have subaccount Member ID emails.
Heads up: If you created a free AT&T email account after June 12, 2011, and you're not an AT&T Internet or DSL customer, you can't create subaccounts.