Microsoft's Email Filters Bounce Legitimate Mail From Senders Without DMARC

Since November 5, Microsoft's filters have rejected some mail to Outlook, Hotmail, Live, and MSN consumer addresses from small senders that lack a DMARC record, with 550 5.7.1 and 550 5.7.23 non-delivery reports. The bounces are inconsistent, and Microsoft has published no policy change, postmaster notice, or service-health entry. The rule requiring DMARC has applied only to bulk senders above 5,000 messages a day since May 2025.

Why it matters: IT and messaging teams should check their sending domains' SPF, DKIM, and DMARC records now and watch for unexplained bounces, since Microsoft has not documented the change or the error.