550 5.7.509 Access denied, sending domain does not pass DMARC verification and has a DMARC policy of reject
Microsoft 365 rejected mail that failed DMARC
Seen in Microsoft 365, Outlook.com
The From domain failed DMARC (neither SPF nor DKIM passed and aligned) and the domain's DMARC policy is p=reject, so Microsoft honoured it.
Common causes
- A sending service (CRM, newsletter, helpdesk) is not in SPF and does not sign with DKIM for your domain.
- SPF passes, but for the provider's bounce domain rather than your From domain, so it does not align.
- A forwarder or mailing list modified the message and broke the DKIM signature.
How to fix it
- Enable DKIM signing with your own domain at every service that sends as you.
- Add the service to SPF, or use a custom return-path on your domain so SPF aligns.
- Use DMARC aggregate reports to find unauthenticated sources before keeping p=reject.
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- 421 4.7.27 Your email has been rate limited because SPF authentication didn't pass for this message · Gmail deferred or blocked mail that failed SPF
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- 550 5.1.1 User unknown · The recipient address doesn't exist