Microsoft Confirms Outlook and Hotmail Blocked Legitimate Emails for Nearly 3 Days

Microsoft has confirmed that a spam filtering bug blocked legitimate emails sent to Outlook.com, Hotmail.com, and Live.com addresses for nearly three days.

The issue caused email delivery failures and SPF validation errors, preventing some messages from reaching their intended recipients.

According to Microsoft incident report EX1545193, the disruption lasted from November 6 to November 9, 2026, before Microsoft resolved it.

Outlook spam filter incorrectly rejected legitimate emails

Affected senders received intermittent non-delivery reports containing error codes 550 5.7.1 and 550 5.7.23, indicating Sender Policy Framework (SPF) validation failures.

Instead of placing legitimate messages in recipients’ spam folders, Outlook’s filtering system rejected them entirely. As a result, recipients never received the affected emails.

Microsoft confirmed that the issue began on November 6 at 18:00 UTC and continued until November 9 at 11:02 UTC, lasting approximately 65 hours.

Microsoft identifies the cause and confirms a fix

Microsoft’s investigation found that a recent update to its spam enforcement rules incorrectly classified legitimate emails as spam and blocked their delivery.

The company has since corrected the issue and restored normal email delivery.

Microsoft also plans to review its update pipelines and deployment processes to prevent similar problems. It will improve its email detection logic to reduce false positives and avoid rejecting legitimate messages.

The incident follows other Microsoft service disruptions, including an Exchange Online and Admin Center outage, a widespread Outlook outage, and a nationwide Microsoft 365 outage that affected Teams.

Via Neowin