I have solved this problem for several computers in my business.  The fix can be found at:

http://www.att.com/esupport/article.jsp?sid=KB401570&cv=804&title=Email+server+settings+(POP+and+SMT...

http://www.att.com/esupport/article.jsp?sid=KB401568#fbid=il0q34wemCK

Although AT&T claims that the old settings should still work,  they do not always work.  Changing the inbound and outbound server addresses has fixed this problem every time.  You should restart your computer after making these changes.

My question is why did AT&T not notify users of the configuration change.  I know that I did not receifve an e-mail message notifying me of the change.