Mailgun outage, September 2026: Microsoft Incident Resulting in Delivery Issues to M365

Mailgun reported and resolved an incident: Microsoft Incident Resulting in Delivery Issues to M365. It lasted 34 minutes.

Resolved

From Mailgun's official status page, in their words.

What happened

Started
Resolved
Duration
34 minutes
Impact
none
  1. Investigating
  2. Identifiednot posted
  3. Monitoring
  4. Resolved

Why it happened

Mailgun has not given a cause in its updates. We never guess one.

How it was resolved

Microsoft has resolved the underlying issue, we are seeing deliveries to M365 process normally.

Mailgun's closing update

This incident on the Mailgun status page

Timeline

Oldest first, from the first notice to the resolution. From Mailgun's official status page, in their words.

  1. monitoring

    Starting around 08:00 UTC, Microsoft began experiencing an incident causing an increased volume of deferred emails across all M365 regions. The affected responses are returning "451 4.7.500 Server busy". Our teams are actively monitoring the situation. Sending to other email providers is not affected.

  2. monitoring

    We are continuing to monitor for any further issues.

  3. resolved

    Microsoft has resolved the underlying issue, we are seeing deliveries to M365 process normally.

What this means for a business (and what to check now)

  • Retry anything that failed during the 34 minutes.
  • Enter any payments, bookings or jobs you took another way.
  • If a deadline fell inside the window, keep this page as your record of the outage.

The full guide: what to do when a service you rely on is down

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