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

We've completed our validation and have started to deploy the change to the affected environments. We anticipate users should start to experience relief within the next 30 minutes.

This quick update is designed to give the latest information on this issue.

We've completed development of an additional fix which is undergoing validation prior to deployment. We expect to complete this process within the next 60 minutes.

This quick update is designed to give the latest information on this issue.

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

User impact: Users may encounter delays receiving external email messages in Exchange Online.

More info: Impact is specific to Simple Mail Transfer Protocol (SMTP), and users may encounter delays of up to 24 hours. Additionally, the email sender may receive 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 be delivered up to 24 hours later.

Current status: We've determined that our previous developed fix to resolve the throttling issue has proven to be unsuccessful. We're developing a new fix that will undergo internal validation to ensure it resolves the underlying issue before deploying it to the impacted environments.

Scope of impact: Impact is specific to some users who are served through the affected infrastructure.

Start time: Monday, October 9, 2023, at 9:00 PM UTC

Root cause: A recent service update, applied to a section of infrastructure responsible for enforcing IP address anti-spam rules, contains a change which is causing impact.

Next update by: Wednesday, October 11, 2023, at 6:30 PM UTC

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

User impact: Users may encounter delays receiving external email messages in Exchange Online.

More info: Affected users may see a "451 4.7.500 Server busy" error message.

Current status: We’re progressing with the testing and development stage of our fix, to ensure its efficacy prior to deployment. We expect to be able to provide an estimate time of completion by the time of the next scheduled update.

Scope of impact: Impact is specific to some users who are served through the affected infrastructure.

Start time: Monday, October 9, 2023, at 9:00 PM UTC

Root cause: 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.

Next update by: Wednesday, October 11, 2023, at 4:30 PM UTC

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

User impact: Users may encounter delays receiving external email messages in Exchange Online.

More info: Affected users may see a "451 4.7.500 Server busy" error message.

Current status: 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.

Scope of impact: Impact is specific to some users who are served through the affected infrastructure.

Start time: Monday, October 9, 2023, at 9:00 PM UTC

Root cause: 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.

Next update by: Wednesday, October 11, 2023, at 2:30 PM UTC

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

User impact: Users may encounter delays receiving external email messages in Exchange Online.

More info: Affected users may see a "451 4.7.500 Server busy" error message.

Current status: We’ve identified that specific IP addresses are being unexpectedly limited by our anti-spam procedures, causing inbound external email delivery to become throttled and delayed. We're reviewing if there have been any recent changes to our anti-spam rules to understand why the IP addresses are being limited. In the meantime, we're manually adding reported affected IP addresses to an allowed list to provide immediate relief.

Scope of impact: Impact is specific to some users who are served through the affected infrastructure.

Next update by: Wednesday, October 11, 2023, at 12:30 PM UTC

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

User impact: Users may encounter delays receiving external email messages in Exchange Online.

More info: Affected users may see a "451 4.7.500 Server busy" error message.

Current status: We’re analyzing sample throttling IPs from simple messages to confirm whether the issue with the portion of SQL infrastructure is causing impact, before we begin formulating a remediation plan.

Scope of impact: Impact is specific to some users who are served through the affected infrastructure.

Next update by: Wednesday, October 11, 2023, at 11:00 AM UTC

Title: Users are unable to receive external email messages within Exchange Online

User impact: Users are unable to receive external email messages within Exchange Online.

More info: Some users email messages may send; however, users may experience delays up to one hour.

Current status: Our review of simple messages has lead us to a potential root cause where a portion of SQL infrastructure may be receiving an unusually large number of requests that may be hindering Exchange Online's performance. We're working to confirm our findings to cultivate further troubleshooting steps.

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.

Next update by: Wednesday, October 11, 2023, at 8:30 AM UTC

Title: Users are unable to receive external email messages within Exchange Online

User impact: Users are unable to receive external email messages within Exchange Online.

Current status: We're currently reviewing simple messages to refine our understanding of this specific impact scenario and establish our troubleshooting steps.

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.

Next update by: Wednesday, October 11, 2023, at 6:30 AM UTC

Title: Users are unable to receive external email messages within Exchange Online

User impact: Users are unable to receive external email messages within Exchange Online.

Current status: We're investigating a potential issue and checking for impact to your organization. We'll provide an update within 30 minutes.