Mail not working.

Have not the AT&T email accounts been converted to Yahoo?

 

Don't take my word for this, maybe someone more knowledgable can chime in. But I think if you delete your mail .plist file, which may be somehow corrupted, it will create a new one next time you use Mail.

I had similar problems with OS X Tiger and searched the internet and found that solution, and it worked for me. But as I say, I'm no expert and possibly someone can confirm (or refute) this as a solution.

 

This is somewhat a true way to solve your problem.
Many people do say you have to remove the plist files of the application if you have disabilities in the application.