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Microsoft Exchange Online mail flow disruption

Service Disruption
First reported
Last updated
Happening score
H score 25
1 unique sources, 1 articles

Summary

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Microsoft is addressing a widespread Exchange Online service disruption that is delaying and failing email delivery for customers across North America and Germany. The issue is causing SMTP deferral errors and interrupted send/access attempts, with some messages left undelivered for over an hour. The outage is still under investigation, so mail flow reliability remains degraded while engineers work to identify the root cause.

Related Happenings

Microsoft Exchange CVE-2026-42897 mitigation advisory

Advisory/Mitigation
First: 15.05.2026 12:40 Last: 15.05.2026 12:40 Sources 1

About this happening: **Microsoft** issued immediate mitigation guidance for **CVE-2026-42897**, reducing risk for **Exchange Server 2016, 2019, and Subscription Edition (SE)** on-premises servers that...

Latest development: 15.05.2026 15:35

Microsoft issued temporary mitigation guidance for CVE-2026-42897 while a patch is still in development, recommending the Exchange Emergency Mitigation (EM) Service, which is enabled by default and can be checked with the Exchange Health Checker script, or the Exchange On-premises Mitigation Tool (EOMT) for disconnected or air-gapped environments. Microsoft noted that the mitigations can disrupt features such as OWA Print Calendar and Inline images, and that servers older than March 2023 cannot receive new mitigations through EM Service.

Open Happening

Microsoft Exchange Server spoofing/XSS flaw under active exploitation (CVE-2026-42897)

Vulnerability
First: 15.05.2026 09:19 Last: 15.05.2026 09:19 Sources 1

About this happening: **CVE-2026-42897** is an **actively exploited** **spoofing/XSS** flaw in **on-premises Microsoft Exchange Server** that can let attackers trigger **arbitrary JavaScript** in a bro...

Open Happening

Microsoft Outlook.com outage causing sign-in failures

Service Disruption
First: 27.04.2026 15:03 Last: 27.04.2026 15:03 Sources 1

About this happening: Microsoft's **Outlook.com** is experiencing an **ongoing outage** that is blocking sign-ins and mailbox access, leaving some customers unable to use email normally. The disruption...

Open Happening

Microsoft Classic Outlook email sending disruption

Service Disruption
First: 02.04.2026 12:12 Last: 02.04.2026 12:12 Sources 1

About this happening: **Microsoft** is investigating a **Classic Outlook** disruption that can prevent some users from sending or replying to email through **Outlook.com**, causing **NDR errors** and m...

Latest development: 06.04.2026 22:19

Microsoft fixed a known issue affecting some Classic Outlook users sending emails via Outlook.com that could return non-delivery reports (NDRs) with 0x80070005-0x0004dc-0x000524 errors. Microsoft said the service change was in production as of April 3, 2026, and advised affected users to use the New Outlook client or Outlook.com on the web; Microsoft also pointed users to downloading the Outlook Address Book for affected Outlook.com accounts.

Open Happening

Microsoft Exchange Online mailbox access disruption

Service Disruption
First: 23.03.2026 14:17 Last: 23.03.2026 14:17 Sources 1

About this happening: The **Exchange Online** service is undergoing an **ongoing mailbox access disruption** that can stop some users from opening cloud mailboxes through **Outlook mobile** and the **n...

Open Happening

Timeline

  1. 02.06.2026 20:02 2 articles · 1h ago

    Microsoft acknowledges Exchange Online mail flow disruption

    Initial Disclosure

    Microsoft acknowledged a widespread Exchange Online mail flow disruption affecting customers in North America and Germany, said the issue is tracked as EX1331830 and classified as an incident, and said engineers are investigating SMTP deferral errors and abrupt connection closures that are delaying or preventing email send and receive operations, with some messages remaining undelivered for over an hour.

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