AT&T is having RBL issues again, we are working with them to resolve the problem. You may receive this problem when emailing the AT&T network for the time being:

<<< 521-65.99.255.236 blocked by sbc:blacklist.mailrelay.att.net.

<<< 521 DNSRBL: Blocked for abuse. See http://att.net/blocks

554 5.0.0 Service unavailable

We have put in place a workaround and are working with AT&T to resolve the issue. You should not continue to see this problem. However, the issue is still open.