Somehow we've triggered an AT&T spam block
The following is in our mail logs:
<firstname.lastname@example.org>: host scc-mailrelay.att.net[220.127.116.11] said:
521-18.104.22.168 blocked by sbc:blacklist.mailrelay.att.net. 521 DNSRBL:
Blocked for abuse. See http://att.net/blocks (in reply to MAIL FROM
That leads you to http://www.att.net/general-info/521.html where hopefully you can see the message that triggered the block.
Unfortunately, http://www.att.net/general-info/521.html is down, so I have no idea what the problem is.
I have submitted a request to AT&T to remove the block, but they said it can take 2 days.