Gmail's Mail Fetcher utility makes it easy in many cases to transfer email from your old AT&T account into your Gmail account. The only catch is that you must have created your AT&T account on or before June 11, 2011; otherwise, AT&T won't give you the POP3 server access required to make the direct transfer.
Sign in to your Gmail account, click the gear icon on the toolbar and select "Settings."
Select the "Accounts" tab, and then click "Add a POP3 Mail Account You Own" in the Check Mail From Other Accounts section.
Enter your AT&T email address, click "Next Step," and then enter your AT&T password.
Enter "inbound.att.net" in the POP Server field, and change the Port setting to "995".
Check the box next to "Leave a Copy of Retrieved Messages on the Server" to prevent Gmail from deleting the emails.
Check the box next to "Always Use a Secure Connection (SSL) When Retrieving Mail."
Check the box next to "Label Incoming Messages" if you want Gmail to mark the messages as having come from your AT&T account. You will be able to access the messages from a separate folder in Gmail if you select this option.
Check box next to "Archive Incoming Messages" if you want to prevent the AT&T messages from appearing in your Gmail inbox.
Click "Add Account" to begin transferring your AT&T mail to Gmail. The process takes several minutes or longer, depending on how much mail you have in your AT&T account.