I’ve just debugged an issue where someone was trying to send an email to my domain, and got back an error message, which they took to mean that the email address no longer existed.
Of the very little debugging information I managed to get out, was this snippet (some information anonymised):
Reporting-MTA: dns;blu0-omc2-s22.blu0.hotmail.com Received-From-MTA: dns;BLU168-W101 Arrival-Date: Fri, 8 Feb 2013 19:04:02 -0800 Final-Recipient: rfc822;email@example.com Action: failed Status: 5.5.0 Diagnostic-Code: smtp;550 relay not permitted
If you search for "hotmail 550 relay not permitted", you will find this post, which describes the almost unbelievable problem.
Turns out hotmail will try to send mail to port 25 of the hosts configured as your A records first, and if it manages to make a connection, will completely ignore your MX records.
I checked the mail server logs on my web server (I do run a mail server there, for other reasons unrelated to that domain), and discovered, yes, there was an attempted mail delivery:
2013-02-09 14:04:26 H=blu0-omc2-s22.blu0.hotmail.com [126.96.36.199] F=<firstname.lastname@example.org> rejected RCPT <email@example.com>: relay not permitted
How anyone could think this was a good idea defies belief. I think I’m still in shock.