IP Blocking Message

Hello,
I have now more than 3 months waiting for an answer, I wrote several times to abuse_rbl@abuse-att.net but they never answer. I dont send SPAM, but the IP of the server is block. I have try to fill the forms http://rbl.att.net/block_inquiry.html and http://rbl.att.net/end_user_request2.html but they are not longer available. Please , I need help seriously.
All my emails are been block and I am getting this error.

For security reason I will erase the ip and the emails.

-----Mensaje original-----
De: Mail Delivery System [mailto:Mailer-Daemon@myservername.com]
Enviado el: Monday, May 22, 2017 2:52 PM
Para: myemail@mydomain.com
Asunto: Mail delivery failed: returning message to sender

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

myfriendemail@bellsouth.net
host ff-ip4-mx-vip1.prodigy.net [144.160.159.21]
SMTP error from remote mail server after MAIL
FROM:< myemail@mydomain.com>:
553 5.3.0 flph397 DNSBL:ATTRBL 521< MY IP >_is_blocked.For assistance forward this email to abuse_rbl@abuse-att.net

Waiting for your help.
Thanks!