Microsoft update misclassifying Fastmail emails to Outlook addresses

Update timeline

1. resolved Jan 27, 2026, 04:30 AM UTC

   Type: Incident Duration: 3 days and 53 minutes Affected Components: Mail delivery Jan 27, 04:30:03 GMT+0 - Identified - A recent Microsoft update is sometimes incorrectly flagging some emails from several providers, including Fastmail. This is causing intermittent delivery issues for some email sent to addresses hosted on [outlook.com](http://outlook.com). We are seeing a small percentage of outgoing mail affected, with no obvious reason as to why. Our team is actively working with Microsoft's email team to get this resolved as soon as possible. Feb 5, 12:00:00 GMT+0 - Monitoring - Microsoft has addressed this issue and we believe the is now resolved. We continue to monitor. Feb 13, 20:30:00 GMT+0 - Identified - We’ve noticed that a small percentage of emails sent to Outlook-hosted addresses are being affected again. We’re in touch with Microsoft and working with them to resolve it. Feb 18, 05:29:00 GMT+0 - Monitoring - The deliverability issues affecting emails sent to Outlook-hosted addresses have stabilized. We are continuing to monitor. Mar 2, 05:23:20 GMT+0 - Resolved - The deliverability issues to Outlook-hosted email addresses are now resolved.