Users may experience delays sending and receiving email messages from external domains in Exchange Online

A post-incident report has been published.

A post-incident report has been published.

Title: Users may experience delays sending and receiving email messages from external domains in Exchange Online User impact: Users may have experienced a delay in sending and receiving email messages from external domains in Exchange Online. More info: Users may have experienced intermittent “Server busy” errors, affecting multiple mailboxes. Depending on the client, the service should attempt to periodically retry sending the email previously impacted by the issue. If the initial sender received an NDR after 24-72 hours (different clients or email providers can vary), they will need to send a new email. Final status: We've confirmed that the issue is resolved after an extended period of monitoring, and receiving additional confirmations from previously impacted organizations. Scope of impact: This issue impacted users who may have been attempting to send and receive email messages from external domains in Exchange Online. Start time: Friday, September 4, 2026, at 9:00 AM UTC End time: Friday, September 4, 2026, at 3:15 PM UTC Root cause: Exchange Online leverages some Microsoft Defender for Office 365 data, along with a multitude of other data sources, for some of its anti-spam features. An upstream distributed data platform experienced elevated service load after a recent change increased the size of read requests beyond the intended physical thresholds. This caused high latency and instability within the platform, which resulted in time windows where portions of the source data were unavailable to dependent processing jobs. One of the sender-based reputation processing jobs continued to run with substantially less input data than expected and still reported successful completions. Because the pipeline did not have a sufficiently strong completeness check before publishing its output, the reduced data set was incorporated into the reputation information used by the mail-protection service. This resulted in incomplete reputation-based data and caused legitimate sending IP addressed to be assigned overly restrictive limits by a volume-based reputation control. When the limits were reached, affected emails were throttled and temporarily deferred with ‘451 4.7.500 Server Busy’ responses. Next Steps: - For a more comprehensive list of next steps and actions, please refer to the Post Incident Report document. We'll provide a preliminary Post-Incident Report within two business days and a final Post-Incident Report within five business days.

Telemetry is trending in a positive direction and shows a reduction in the number of new emails getting impacted as the rule disablement progresses across the service. Next update by: Friday, September 4, 2026, at 4:30 PM UTC This quick update is designed to give the latest information on this issue.

We've determined that a different machine level throttling rule within the anti-spam features is being incorrectly triggered for a small number of emails, resulting in the remaining impact. We're disabling this specific rule to provide relief and fully mitigate the issue. We expect this to complete within the next hour. Next update by: Friday, September 4, 2026, at 4:30 PM UTC This quick update is designed to give the latest information on this issue.

Title: Users may experience delays sending and receiving email messages from external domains in Exchange Online User impact: Users may experience a delay in sending and receiving email messages from external domains in Exchange Online. More info: Users may experience intermittent “Server busy” errors, affecting multiple mailboxes. Depending on the client, the service should attempt to periodically retry sending the email previously impacted by the issue. If the initial sender received an NDR after 24-72 hours (different clients or email providers can vary), they will need to send a new email. Current status: We've received a large number of confirmations that the issue is resolved, however, we're reviewing some isolated reports that some organizations are still experiencing delays in email delivery. We're reviewing example SMTP traces from those impacted users to isolate the source of the issue. Scope of impact: This issue impacts users who may be attempting to send and receive email messages from external domains in Exchange Online. This information may be updated as our investigation continues. Start time: Friday, September 4, 2026, at 9:00 AM UTC Preliminary root cause: The enforcement of a specific anti-spam model is contributing to impact for a subset of email traffic. Next update by: Friday, September 4, 2026, at 4:30 PM UTC

Title: Users may experience delays sending and receiving email messages from external domains in Exchange Online User impact: Users may experience a delay in sending and receiving email messages from external domains in Exchange Online. More info: Users may experience intermittent “Server busy” errors, affecting multiple mailboxes. Depending on the client, the service should attempt to periodically retry sending the email previously impacted by the issue. If the initial sender received an NDR after 24-72 hours (different clients or email providers can vary), they will need to send a new email. Current status: Telemetry shows that the service is performing as expected for new emails and this is in line with organizations confirming that the issue no longer occurring. We're monitoring email queues and performing final validations to confirm the issue is fully resolved. Scope of impact: This issue impacts users who may be attempting to send and receive email messages from external domains in Exchange Online. This information may be updated as our investigation continues. Start time: Friday, September 4, 2026, at 9:00 AM UTC Preliminary root cause: The enforcement of a specific anti-spam model is contributing to impact for a subset of email traffic. Next update by: Friday, September 4, 2026, at 3:00 PM UTC

Some organizations are reporting that they are experiencing relief and that their queued messages are now processing. We continue to monitor the service to confirm full resolution. Next update by: Friday, September 4, 2026, at 1:30 PM UTC This quick update is designed to give the latest information on this issue.

Title: Users may experience delays sending and receiving email messages from external domains in Exchange Online User impact: Users may experience a delay in sending and receiving email messages from external domains in Exchange Online. More info: Users may experience intermittent “Server busy” errors, affecting multiple mailboxes. Current status: We're progressing with the disablement of the specific model, and telemetry is showing a decrease in matching and throttling actions as the rule changes propagate throughout the service. We estimate this rollout will complete within the next 30 minutes. Scope of impact: This issue impacts users who may be attempting to send and receive email messages from external domains in Exchange Online. This information may be updated as our investigation continues. Next update by: Friday, September 4, 2026, at 1:30 PM UTC

Title: Users may experience delays sending and receiving email messages from external domains in Exchange Online User impact: Users may experience a delay in sending and receiving email messages from external domains in Exchange Online. More info: Users may experience intermittent “Server busy” errors, affecting multiple mailboxes. Current status: We've identified that the enforcement of a specific anti-spam model is contributing to impact for a subset of email traffic. We're adding affected IPs from the throttling enforcement to an allow list as an initial mitigation measure. In parallel, we're working to disable the specific model that caused impact. Scope of impact: This issue impacts users who may be attempting to send and receive email messages from external domains in Exchange Online. This information may be updated as our investigation continues. Next update by: Friday, September 4, 2026, at 2:30 PM UTC

Title: Users may experience delays sending and receiving email messages from external domains in Exchange Online User impact: Users may experience a delay in sending and receiving email messages from external domains in Exchange Online. More info: Users may experience intermittent “Server busy” errors, affecting multiple mailboxes. Current status: Our initial investigation has identified instances where anti-spam protections may be contributing to impact for a subset of users. We are continuing to analyse the behaviour and review service telemetry to better understand the underlying cause and determine the most effective mitigation path. Scope of impact: This issue impacts users who may be attempting to send and receive email messages from external domains in Exchange Online. This information may be updated as our investigation continues. Next update by: Friday, September 4, 2026, at 12:30 PM UTC

Title: Users may experience delays receiving email messages from external domains in Exchange Online User impact: Users may experience a delay in receiving email messages from external domains in Exchange Online. More info: Users may experience intermittent “Server busy” errors, affecting multiple mailboxes. Current status: We're reviewing service monitoring telemetry to isolate the source of the issue and establish a fix. Scope of impact: This issue impacts users who may be attempting to receive email messages from external domains in Exchange Online. This information may be updated as our investigation continues. Next update by: Friday, September 4, 2026, at 12:30 PM UTC

Title: Users may experience delays receiving email messages from external domains in Exchange Online User impact: Users may experience a delay in receiving email messages from external domains in Exchange Online. More info: Users may experience intermittent “Server busy” errors, affecting multiple mailboxes. Current status: We're investigating a potential issue and checking for impact to your organization. We'll provide an update within 60 minutes.