Why Are Some of My Messages Failing Authentication Into Outlook.com?

Problem:

My email is hosted by Microsoft (Office 365 or Exchange Online Protection) and I have spf.protection.outlook.com in the SPF record for my domain but some mail still fails SPF authentication; how can that be?

Solution:

Based on the method Microsoft has chosen to implement SPF validation as well as DMARC support for inbound email on Office 365 and Exchange Online Protection, email leaving your organization BUT destined for another O365/EOP customer, you must have your originating server IP address in the domain's SPF record.  It may also be necessary to publish an SPF record for each server to cover bounce (NDN/NDR) messages.

 

More information is available here:

http://blogs.msdn.com/b/tzink/archive/2015/02/26/how-office-365-does-spf-checks-for-customer-to-customer-mail.aspx 

 

https://technet.microsoft.com/en-us/library/dn789058%28v=exchg.150%29.aspx

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