In an effort to cut down on spam, AT&T by default blocks pretty much all external smtp servers.
You can remove this block by submitting an abuse report here: http://helpme.att.net/servabuse.php
Choose Opt-out Port 25 from the abuse type list.
They recommend that mail hosts use an authenticated port 587 instead of 25, so you wouldn't need to worry about the port block.
(As for reaching work smtp from at&t? You can always use vpn, no?)