Legitimate email from our MX to
@attglobal.net addresses is being blocked.
Their MX rejects the mail with this error message (IP address redacted):
550 unauthorized interface for x.x.x.x on kcin04
We have correct MX and PTR records and no current blacklists hits. The only explanation of what this error message might mean I found on the generic AT&T error message page, which says that the error indicates that
The sending system is not recognized as a valid source of e-mail.
Correspondents should use registered mail systems provided by a major ISP or hosting service.
This server has been in operation for years, and outgoing mail is accepted by the likes of Gmail, Yahoo, Hotmail, etc. How could it not be a "valid source of e-mail"? And what's a "registered mail system" anyway? And does all of this even apply to
attglobal.net, or does the generic AT&T page only apply to their other subsidiaries? In which case, how do I find out why we're being blocked?
I did try submitting the block removal request form, but the response I got back was:
We do not show that the IP address(es) listed in your request are on AT&T’s block list
Also, email to
@bellsouth.net addresses appear to be transmitted without problems, so only
attglobal.net seems to be blocking.