I have one domain, let's call it example.com with 2 MX records.
The first record points to my on site Exchange 2013 server with a priority of zero, making it the preferred server. The second record points to my hosting provider's mail server with a priority of 10.
To my understanding this should render the second mail server as a backup server. I have a POPcon downloading the mails to my exchange server. Recently my DSL link on site has gone down causing my Exchange 2013 to NOT be available from the internet, which should have caused the mails to be delivered to the Hosting providers mail server. During this downtime period all incoming mails bounced with the following error message;
Reporting-MTA: dns;BLU004-OMC4S16.hotmail.com Received-From-MTA: dns;BLU437-SMTP62 Arrival-Date: Mon, 26 Jan 2015 02:09:28 -0800 Final-Recipient: rfc822;firstname.lastname@example.org Action: failed Status: 5.5.0 Diagnostic-Code: smtp;550-Please turn on SMTP Authentication in your mail client, or login to the 550-IMAP/POP3 server before sending your message. blu004-omc4s16.hotmail.com 550-[22.214.171.124]:64110 is not permitted to relay through this server without 550 authentication.
After deleting my exchange 2013 MX record from DNS, all mails started flowing normally.
What causes this unexpected behaviour?