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;user@example.com
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-[]: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?

  • 1
    Looks like you haven't configured your backup server properly.. – NickW Jan 28 '15 at 12:09
  • Thanks for you response NickW, would you please be more specific, what kind of config are you referring to? Like I said, after deleting the MX record for my onsite Exchange server, all mail was delivered to my hosting providers mail server. – Andre Carstens Jan 28 '15 at 12:12
  • Basically, your provider's mail server is looking up the record for your domain, and since it sees your Exchange server, it assumes it has to relay (and you get the error). I have no idea what your provider's server is, but it needs to be configured to accept mail for your domain regardless, then forward it when your server is back online.. – NickW Jan 28 '15 at 12:16

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