Office 365 Exchange Online Protection and DMARC

If you want to protect your domain with DMARC or use DMARC to filter spam and you use Office 365, note that Microsoft decided to alter normal DMARC policy. Imagine the domain protects itself and a message was identified as DMARC=fail and policy is set to reject with 100%. DMARC policy example: v=DMARC1;p=reject;pct=100
Office 365 will ignore reject and will deliver email marked as spam. A header will contain

"dmarc=fail action=oreject" (oreject being overwritten reject.)

Here is how Microsoft justifies this design decision:

"If the DMARC policy of the sending server is p=reject, EOP marks the message as spam instead of rejecting it. In other words, for inbound email, Office 365 treats p=reject and p=quarantine the same way.

Office 365 is configured like this because some legitimate email may fail DMARC. For example, a message might fail DMARC if it is sent to a mailing list that then relays the message to all list participants. If Office 365 rejected these messages, people could lose legitimate email and have no way to retrieve it. Instead, these messages will still fail DMARC but they will be marked as spam and not rejected. If desired, users can still get these messages in their inbox through these methods:

• Users add safe senders individually by using their email client
• Administrators create an Exchange mail flow rule (also known as a transport rule) for all users that allows messages for those particular senders."

https://docs.microsoft.com/en-us/office365/securitycompliance/use-dmarc-to-validate-email#inbounddmarcfail