IP Address Blacklisted

Hello,

My company has been recently blacklisted. 

All emails we send to AT&T and it's related domains are bouncing with:

This is an automatically generated message from SendGrid.

I'm sorry to have to tell you that your message was not able to be delivered to one of its intended recipients.

If you require assistance with this, please contact SendGrid support.

*redacted*: 553 5.3.0 flpd584 DNSBL:RBL 521< xxx.xxx.xxx.xxx >_is_blocked.For assistance forward this email to abuse_rbl@abuse-att.net

We are a cinema chain that uses a static sender IP from SendGrid to mail movie tickets and gift cards. 

The blacklisting is causing major issues for us. I can provide specific details for the sending IP to an AT&T rep.

Our sending IP is only blacklisted by AT&T. All other providers report us as a clean sender.

-DT