We've just forwarded a spam report to the Bell South (part of AT&T) abuse address and got this for our trouble:

abuse@REDACTED SMTP error from remote mail server after initial connection: host gateway-f1.isp.att.net [204.127.217.16]: 550-77.74.196.254 blocked by ldap:ou=rblmx,dc=att,dc=net 550 Error - Blocked for abuse. See http://att.net/blocks