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:
<<< 521-184.108.40.206 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.