repeated email blocks

Please forward to ATT E-mail security people:

I manage a Virtual Private Server at DigitalOcean.  For the past month, I have had outgoing mail blocked by ATT with a 553 error.

During this time ,I have investigated, and repaired accounts.  I have rebuilt the server, using a different assigned IP, about 4 times.

Today, my server at newly assigned 67.205.149.169 was blocked immediately upon startup.

Each time the server is rebuilt, I attempt to send a message from a hosted acct (***) to my ATT acct (***).  I always get a bounce message like:

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:

  ***   host ff-ip4-mx-vip2.prodigy.net [144.160.159.22]

   SMTP error from remote mail server after MAIL FROM:<***>:

   553 5.3.0 flpd589 DNSBL:ATTRBL 521< 67.205.149.169 >_is_blocked.For assistance forward this email to abuse_rbl@abuse-att.net

I do forward the message; seldom get a response; and never get a reason or resolution to the problem.  MxToolbox and other rbl monitors do not show errors with the site.  If I could, at least, know WHY the site is getting blocked, I can take some action.

FYI - Server is on CentOS 7, using CloudLinux, and cPanel.  Server is in what DigitalOcean calls a Droplet of virtual hosting with attached Volumes of storage.  Each rebuild takes me 1-2 days. 

[Edited for privacy. This is a public forum. Please do not post personal or unique information such as but not limited to full names, employee ID numbers, email addresses, phone numbers, account numbers, etc.]