ATA is not currently hiring, but they should be soon, as they are seriously short of FOs. You can try snail mail at:
American Trans Air
Chief Pilot's Office
P.O. Box 51609
Indianapolis, Indiana 46209
The Chief Pilot's name is Dave Lindskoog.
Also you can fax at 317 282-5636
If you tell anyone where you got this fax number , I'll admit nothing, deny everything, and make counter accusations. Good luck.