Microsoft 365 Inbound Email Delays

By Engineering Team In General On October 11, 2023

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Currently there is an ongoing Microsoft incident affecting inbound email delivery to Microsoft Exchange online, this may result in inbound email delays if your email is hosted with 365 and may cause delays sending if you are sending to recipients who’s emails system is hosted on 365.

Below update from Microsoft:

“11 October 2023 at 19:25 BST

Title: Some users may encounter delays sending and receiving external email messages in Exchange Online

User impact: Users may have encountered delays sending and receiving external email messages in Exchange Online.

More info: Impact was specific to Simple Mail Transfer Protocol (SMTP). Most users would have experienced up to a 1-hour delay; however, in some cases, users may have encountered delays of up to 24 hours. Additionally, the email sender may have received a Non-Delivery Report (NDR) with a “451 4.7.500 Server busy” error message, and their attempt to retry delivery of the email may have been delivered up to 24 hours later.

Final status: We’ve completed our deployment of the change, which was a targeted reversion of the code issue causing impact. Users should now be experiencing mitigation, though some residual delays may still occur over the next hour as the change completes.

Scope of impact: Impact was specific to some users who were served through the affected infrastructure. The issue affected users Worldwide and only for a portion of IP addresses, therefore not all users experienced impact.

Start time: Monday, October 9, 2023 at 10:00 PM GMT+1

End time: Wednesday, October 11, 2023 at 6:00 PM GMT+1

Preliminary root cause: A recent service update that was applied to a section of infrastructure responsible for enforcing IP address anti-spam rules contained a code issue, which was resulting in the delays with SMTP email delivery.

Next Steps:

• We’re further reviewing the recent update to understand how the code issue was introduced and to understand what prevented it from being detected in our update testing and validation procedures, which will allow us to prevent similar issues in future updates.

We’ll publish a Post-Incident Report for this event within five business days.”

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