Outlook and Hotmail bounced legit emails for 4 days, and they're gone

What caused the Outlook error 550 5.7.1 bounces?

Microsoft tracked it as incident EX1545193. An update to its own anti-spam rules flagged properly configured domains as junk. Those senders already met the three authentication standards, SPF, DKIM and DMARC, which are checks that an email really comes from the domain it claims. They had nothing to fix on their side.

Since 5 May 2025, Microsoft has required these standards from domains sending more than 5,000 emails a day to its mailboxes. This outage went well beyond that group. Senders far below the threshold were rejected too, with a code that appears in no Microsoft documentation. The company did not spot the problem itself. User reports on help forums, piling up several days after the first rejections, triggered the alert. Only consumer Outlook seems affected, while Microsoft 365 business accounts reportedly were not.

Can the rejected emails be recovered?

No. Messages refused between 5 and 9 November never reached the recipient. They are not in the spam folder, and nothing will bring them back. Only the sender was told, through a non-delivery notice. The only fix is to send each message again, and Microsoft says senders don't need to change any settings first. Rejections were also intermittent: a refused message could go through on a second try 15 minutes later.

This isn't the first time. In late February, Microsoft's servers had already rejected legitimate emails for a full week. The company now promises to review its update and testing procedures so a faulty rule doesn't go live without safeguards.