Announcement - Delays receiving external email messages in Exchange Online

Dear clients,

***Only clients that subscribe to Microsoft 365 will be affected**

User impact: 

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


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.  


Microsoft completed their 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: Tuesday, October 10, 2023 at 5:00 AM GMT+8  

End time: Thursday, October 12, 2023 at 1:00 AM GMT+8  

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.  

Best Regards, 


Support Team
Technical Support Department