Att.net and sbcglobal.net emails undeliverable

Delivery to every recipient with an att.net or sbcglobal.net email address fails with the following message:

host al-ip4-mx-vip1.prodigy.net [144.160.235.143]

SMTP error from remote mail server after MAIL FROM:<[email scrubbed]>:

553 5.3.0 alpd683 DNSBL:RBL 521< 69.16.226.164 >_is_blocked.

I've sent the errors to this email address as instructed: abuse_rbl@abuse-att.net. It's been a week now and is impeding our ability to do business, especially as we are all operating remotely at this time.