Outlook.com intermittently rejects legitimate email
A Microsoft support-forum moderator says the company has classified the delivery problem as a service degradation and is investigating its cause.
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A Microsoft support-forum moderator says the company has classified the delivery problem as a service degradation and is investigating its cause.
Outlook.com is intermittently rejecting legitimate emails, with some messages getting through when senders try again. The disruption affects Microsoft’s consumer email services and began at the end of the preceding week, The Register said in its November 9 report. It described the problem as apparently widespread based on affected users' accounts. Microsoft’s bounce guidance covers Outlook.com and related consumer services including Hotmail, Live.com and MSN.
Some senders received bounce messages citing failed SPF checks or missing DMARC information; others received vague explanations. Forum users said even a single, one-line email could be rejected.
The moderator said Microsoft engineers were examining service data and information from customers to identify the cause. The Register suggested tighter authentication enforcement as a possible explanation, including for smaller senders, but Microsoft had not explained any rule change or responded to its inquiry.
Microsoft announced stronger authentication rules for high-volume senders in 2025. The Register described them as applying to domains sending more than 5,000 emails per day. Microsoft’s support guidance defines a high-volume sender as one sending 5,000 or more messages to its consumer services using the same domain in the visible From address.
Microsoft says the rules aim to reduce spam and phishing. They require SPF and DKIM checks to pass and a DMARC record to be published. SPF checks whether a sending source is authorized for the envelope-sender domain, which can differ from the visible From-address domain; DKIM uses a signature to check a message’s authenticity and integrity. DMARC validation requires at least one of those checks to align with, or effectively match, the domain in the visible From address. Microsoft’s guidance accepts DMARC policies of p=none, p=quarantine and p=reject.
Some customers said mail began flowing again after they adjusted their authentication settings to Microsoft’s requirements. Others said their messages were still rejected even though their SPF, DKIM and DMARC settings complied.
Microsoft’s general guidance recommends checking the bounce message and message headers for authentication results, then verifying domain records, authorized sending sources and alignment. Senders using a third-party service should also check that DKIM signs messages with their domain, that their SPF record includes the service’s required IP addresses or include values, and that DMARC validation uses their domain. Microsoft’s investigation continues; it has not identified the cause.
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