Office 365 mail servers on Spam Blacklists?

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Since we’ve been migrating companies off IBM Lotus Domino we made the decision a couple of years ago to migrate ourselves.  We decided that we no longer wanted to host our own mail servers.  Why should we.  The cost of a cloud based email provider was so cheap..

I’ve long suspected that some of my mail was not being delivered to clients.  Most anti-spam products silently delete mail.  The recipient doesn’t get it and the sender doesn’t get notified.  But I started noticing that some reply messages would sometimes have a warning in the subject line.  Sometimes it was the content of my original mail message being flagged.   For example I use a URL shorten-er to refer to this blog.  That is flagged as a potential threat.

Sometimes it was the sender ip address.  This is the part that I had difficulty with.  I’m no longer the sender.  I’m using a shared service that comes from Microsoft.  So Microsoft is now the sender.  So lets think this through.  I send and receive mail through microsoft services.  Microsoft uses multiple data centres and load balancers to send and receive mail.   Those servers are shared by other companies.  Could other users of the same service be sending out potential spam.  Could their recipients be blocking their mail.

But hold on a second… If their mail is getting flagged as a spam… That means that the office365 IP addresses are getting flagged as spam IP addresses.  Then that means that my mail is also considered spam.

Last night I got a whole bunch of these responses to my email messages:

Your message wasn’t delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept email from certain senders, or another restriction may be preventing delivery. For more tips to help resolve this issue, see DSN code 5.7.1 in Exchange Online – Office 365. If the problem continues, contact your helpdesk.

The problem with this is that you don’t have a lot of control over the Office365 servers.  And you can only take care of this when you get notified.  And even then its out of your control.  You’re at the mercy of Microsoft and when they can re-establish service.  And that’s after they keep telling you that its impossible.

The real problem is that this happens more often the I realize.  This time I was given a message which told me.  How often has it happened with out being notified.




  1. Karl-Henry Martinsson  December 8, 2014

    You could sned yourself a couple of different emails and check the mail headers for the IP addresses. Then you lookup those addresses on one of the free services who check against hundreds of blocklists.

    Or you can try contacting Microsoft to get the range of IP addresses, but I doubt that will be successful.


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