Sending an email to AT&T results in: DNSBL:RBL 521< $IP >_is_blocked

Symptoms

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 user@domain.tld with the email address that sent the email).

SMTP error from remote mail server after MAIL FROM:<user@domain.tld>:
553 5.3.0 alph762 DNSBL:RBL 521< $IP >_is_blocked.
For assistance forward this error to abuse_rbl@abuse-att.net

Description 

This indicates AT&T is blocking emails from the IP address that was used to send the email.

Workaround

Send the error to the email address that is listed in the error. AT&T provides more information here:

AT&T - Postmaster Help