When you set up an AT&T account, you’ll have the option of creating a new AT&T webmail address. AT&T’s webmail service is supported by Yahoo, and is a convenient feature for users without an alternate email address. However, if you already have a Gmail account and you’d prefer your AT&T communications go there, you can tell the AT&T webmail account to forward all of your messages.

How to Automatically Forward Email From an AT T Account to Gmail  6 Steps  - 77