Blog Post

Some users may encounter delays receiving external email messages in Exchange Online

The Issue

Today, Microsoft 365 users are facing a widespread email outage affecting the ability to receive emails from external organizations. The issue is causing significant disruptions in communication for businesses and individuals alike. Emails sent to domains hosted by Microsoft are being delayed and/or bouncing back with the error message “451 4.7.500 Server busy. Please try again later.”

The Cause

According to Microsoft’s Service Health page, the outage was triggered by a recent update:

We’ve identified that a recent service update, applied to a section of infrastructure responsible for enforcing IP address anti-spam rules, contains a change which is inadvertently causing impact. We’re developing and validating a fix to revert to the most recent stable service version to remediate impact. In parallel, we’re continuing to manually add reported affected IP addresses to an allowed list to alleviate impact in the short term.


The outage is causing a ripple effect across various sectors that rely on Microsoft 365 for their email communications. According to Statista, Microsoft 365 is used by over a million companies worldwide and represents about 46% of the market for office suite software. From small businesses to large enterprises, the inability to send emails is affecting productivity and causing delays in critical communications.

What is Microsoft Doing?

Microsoft is aware of the issue and is actively working on a fix. Their engineers are currently investigating the root cause and are likely to roll back the problematic update or issue a new one to resolve the problem. Microsoft has not provided an estimated time for the resolution, but they are updating their Service Health page regularly to keep users informed.

What Can You Do?

While waiting for Microsoft to resolve the issue, here are some steps you can take:

  1. Check Microsoft’s Service Health Page: Keep an eye on updates from Microsoft to know when the issue is resolved.
  2. Use Alternative Communication Channels: Consider using other platforms for urgent communications, such as instant messaging apps or phone calls.
  3. Notify Your Contacts: If possible, inform your contacts about the issue so they are aware that emails may be delayed or not arrive at all.
  4. Reschedule Critical Communications: If you had planned to send important emails today, consider rescheduling them until the issue is resolved.
  5. Draft and Save: Prepare your emails as drafts so that you can send them out as soon as the service is restored.

Customers using MXGuardian Email Security to filter their outbound email can view all messages that have been rejected by Microsoft by going to the Outbound tab and using the dropdown menu to filter by category “Bounced”. Once the issue has been resolved by Microsoft, you can resend all bounced email from the MXGuardian control panel.


The Microsoft 365 email outage is a significant disruption that is affecting a large number of users. While the issue is unfortunate, it serves as a reminder of the vulnerabilities inherent in relying on a single platform for critical business functions. Microsoft is working diligently to resolve the issue, and we can only hope for a speedy resolution. In the meantime, consider using alternative methods of communication to keep your operations running as smoothly as possible.