Outlook.com bounces legit emails as Microsoft's authentication crackdown goes sideways

A validation error has stopped Outlook.com from accepting legitimate emails.

The problem appears widespread and, according to reports from affected users, can result in non-delivery reports with error codes related to SPF (Sender Policy Framework) validation failures.

Other users have reported errors highlighting a lack of DMARC (Domain-based Message Authentication, Reporting, and Conformance) information.

The troubles began at the end of last week and appear to be the result of Microsoft tightening the rules (or at least attempting to do so) for accepting emails. The company's consumer email services are affected, and it appears that if a domain doesn't have working SPF or if it has a missing DMARC record, then Outlook.com and its ilk will treat it as suspicious and not accept emails.

The rules are designed to deal with spam, bulk senders, and the like. In 2025, Microsoft announced new rules designed to strengthen email authentication for domains sending more than 5,000 emails per day. …