Outlook.com rejects email from domains without DMARC: what senders report and how to fix it

Senders report that since early November Outlook.com has bounced some email from domains without a DMARC record with error 550 5.7.1. Microsoft has not officially confirmed the change.

A report that Outlook.com is rejecting mail from domains without a DMARC record collected more than 340 points on Hacker News this week. According to several deliverability blogs it began around 5 November 2026 and affects consumer addresses at outlook.com, hotmail.com, live.com and msn.com. The message bounces with error 550 5.7.1 and text saying it lacks SPF, DKIM or DMARC authentication records.

It matters what is confirmed. Since May 2025 Microsoft has had rules for high-volume senders, meaning 5,000 or more messages a day to its consumer services, that require SPF, DKIM and DMARC with at least a p=none policy. The error for breaking those rules is 550 5.7.515 and is documented on Microsoft's support site. The November reports are different: error 5.7.1 reportedly hits smaller senders too. One blog notes these are so far individual cases and do not establish a new blanket rule. We did not find an official Microsoft announcement.

What should companies do? Create a TXT record at _dmarc on the domain they send from, with policy p=none, and check that SPF or DKIM aligns with the sending domain. According to reports, this cleared the rejection in confirmed cases. Jumping straight to p=reject is not advised, because you could block legitimate mail, for example from an invoicing system or newsletter service.

Is this relevant for Czech companies? Yes. Customers often have hotmail.com or outlook.com addresses, and order confirmations, invoices or support replies that fail to arrive show up as lost revenue and extra support requests. A domain without DMARC is also easier to abuse for fraudulent email in your company's name.