When sending an email to an AT&T email address, the following error is returned (replace $IP with the IP address that was used to send the email and firstname.lastname@example.org with the email address that sent the email).
SMTP error from remote mail server after MAIL FROM:<email@example.com>:
553 5.3.0 alph762 DNSBL:RBL 521< $IP >_is_blocked.
For assistance forward this error to firstname.lastname@example.org
This indicates AT&T is blocking emails from the IP address that was used to send the email.
Send the error to the email address that is listed in the error. AT&T provides more information here: