Outage in Fastmail
Microsoft update misclassifying Fastmail emails to Outlook addresses
Resolved Minor
January 27, 2026 - Started 2 months ago
- Lasted about 1 month
Incident Report
Summary AI Generated
A Microsoft update caused intermittent delivery issues for Fastmail emails sent to Outlook.com addresses, with a small percentage of outgoing mail being incorrectly flagged and blocked. The issue affected mail delivery for over 34 days, recurring even after an initial fix was implemented. Fastmail worked directly with Microsoft's email team to resolve the classification problem, and delivery has now stabilized.
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.
Components affected
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Latest Updates ( sorted recent to last )
IDENTIFIED
2 months ago - at 01/27/2026 04:30AM
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.
MONITORING
about 2 months ago - at 02/05/2026 12:00PM
Microsoft has addressed this issue and we believe the is now resolved. We continue to monitor.
IDENTIFIED
about 2 months ago - at 02/13/2026 08:30PM
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.
MONITORING
about 1 month ago - at 02/18/2026 05:29AM
The deliverability issues affecting emails sent to Outlook-hosted addresses have stabilized. We are continuing to monitor.
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