As far as I can tell the IP address for the server I started renting last year is blacklisted by them. It's clear at all the big spam databases (I checked when I got the server) but despite polite requests ATT continues to reject all email from my sites. As registration and user account management is automated and requires a working email address I've blacklisted email addresses from the few affected ISPs that use ATT as a spam filter. C'est la vie!