Users routing their Exchange Online email through a third-party protection service may have send and receive delays

Title: Users routing their Exchange Online email through a third-party protection service may have send and receive delays

User impact: Users routing their Exchange Online email through a third-party protection service may have send and receive delays.

More info: This issue specifically impacts users who use Simple Mail Transfer Protocol (SMTP) as their connection method and route their email messages through a specific third-party email protection service. The email delays, which impacts both sending and receiving email, occurs in all Exchange Online connection methods.

Current status: Though our investigation into the root cause of this impacting issue is ongoing, we've identified and implemented a configuration change in effort to mitigate the problem. We're reviewing our internal telemetry to assess if the configuration change was effective. Please note that we've updated the Title, User impact, and More info of this communication to better reflect the impact of the event.

Scope of impact: This problem may impact any user sending or receiving email messages when leveraging a specific third-party email protection service.

Next update by: Friday, January 30, 2026, at 8:00 PM UTC

Title: Users may experience delays when sending or receiving Exchange Online email messages

User impact: Users may experience delays when sending or receiving Exchange Online email messages.

More info: This issue specifically impacts users who are on Simple Mail Transfer Protocol (SMTP) connection methods, which include Outlook on the web, the Outlook desktop clients, and the mobile apps.

Current status: Our investigation has further identified that exceptions may be occurring as our mail transport service is sending an unexpectedly large amount of requests to downstream components in the mail routing code path. We're investigating recent changes to confirm this theory and determine next steps for remediation.

Scope of impact: Some users attempting to send or receive Exchange Online email messages may be impacted.

Next update by: Friday, January 30, 2026, at 7:30 PM UTC

Title: Users may experience delays when sending or receiving Exchange Online email messages

User impact: Users may experience delays when sending or receiving Exchange Online email messages.

More info: This issue specifically impacts users who are on Simple Mail Transfer Protocol (SMTP) connection methods, which include Outlook on the web, the Outlook desktop clients, and the mobile apps.

Current status: We've identified the "451 4.3.2 Temporary server error" occurring that's leading to impact. This error, generally, is a common error, however our review of transport service health telemetry confirmed an increase in the rate of this error generating. We're investigating why we see an increase of this error resulting in impact to better understand how the issue is manifesting and prepare next steps for potential remediating actions.

Scope of impact: Some users attempting to send or receive Exchange Online email messages may be impacted.

Next update by: Thursday, January 29, 2026, at 9:30 PM UTC

Title: Users may experience delays when sending or receiving Exchange Online email messages

User impact: Users may experience delays when sending or receiving Exchange Online email messages.

More info: This issue specifically impacts users who are on Simple Mail Transfer Protocol (SMTP) connection methods, which include Outlook on the web, the Outlook desktop clients, and the mobile apps.

Current status: We're reviewing transport service logs in parallel with samples of affected messages to identify and isolate any errors that may be contributing to impact for further investigation.

Scope of impact: Some users attempting to send or receive Exchange Online email messages may be impacted.

Next update by: Thursday, January 29, 2026, at 7:30 PM UTC

Title: Users may experience delays when sending or receiving Exchange Online email messages

User impact: Users may experience delays when sending or receiving Exchange Online email messages.

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