We have a lot of small business using domains sbcglobal.net, which is now part of AT&T. We are having issues with emails getting through. It doesn't matter the email content or from address. They always hard bounce with the error code "553 5.3.0 flph399 DNSBL:RBL 521< XXX.XXX.XXX.XXX >_is_blocked.__For_information_see_http://att.net/blocks". Obviously they are getting blocked. We've limited sending to sbcglobal email address to only-as-needed basis. Had them send an email to us then reply using the outlook plugin so the reply came from our marketo server, which has a dedicated IP. We can send emails without issue through our outlook server.
Have you had issues with sbcglobal? What did you do to fix it?