Outlook.com Is Bouncing Legitimate Emails: Check Your Email Authentication Now

Outlook.com has been rejecting legitimate emails since the end of last week, according to The Register. Affected senders report bounce messages citing SPF (Sender Policy Framework) validation failures, while others see errors about missing DMARC (Domain-based Message Authentication, Reporting, and Conformance) information. The problem appears to be linked to Microsoft tightening, or attempting to tighten, its rules for accepting mail on its consumer email services. Domains without working SPF or with no DMARC record seem to be treated as suspicious.

The rules are aimed at spam and bulk senders. In 2025, Microsoft announced stronger authentication requirements for domains sending more than 5,000 emails a day, covering DMARC, SPF and DKIM (DomainKeys Identified Mail). However, small senders now appear to be affected too, with forum users reporting that even a one-line email can be rejected. Microsoft has not issued an advisory explaining the changes. The fault is also intermittent, so a resend may succeed, and the bounce messages can be vague about the cause. A moderator in Microsoft's support forum acknowledged the bug and classed it as a "service degradation".

Many affected users say email flowed again once they aligned their settings with Microsoft's preferences. SPF must pass for the sending domain, with DNS records accurately listing authorised senders. DKIM must pass to confirm the email's integrity and authenticity. DMARC needs at least a policy of p=none and must align with SPF or DKIM, preferably both. Some customers insist their settings were already compliant. The Register has asked Microsoft for comment but had not received a response.