Sounds like they might be using some sort of Bogon list.
https://en.wikipedia.org/wiki/Bogon_filtering
From: mailop [mailto:mailop-boun...@mailop.org] On Behalf Of David Hubbard
Sent: Tuesday, October 11, 2016 6:21 PM
To: mailop@mailop.org
Subject: [mailop] att.net/blocks issues; how to get
Hi all, we’re an ecommerce host (i.e. web host specific to our own ecommerce
software), and we been having recurring issues getting some IP addresses for
customer servers removed from the AT DNSBL list whose SMTP rejection advises
requesting via att.net/blocks. I saw some talk of similar
