11/10/2023 10:31:00 AM (GMT)
NHSmail Reference : INC38814257
Microsoft Reference : EX680695
Issue Status : RESOLVED
Issue Description : Users may encounter delays 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 Update : 12/10/2023 08:56:00 AM – 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 : Your organization is impacted by this event, as well as users who are attempting to receive external email messages within Exchange Online.
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 : Microsoft are further reviewing the recent update to understand how the code issue was introduced and to understand what prevented it from being detected in their update testing and validation procedures, which will allow them to prevent similar issues in future updates.