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Microsoft Classic Outlook email sending disruption

Service Disruption
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1 unique sources, 2 articles

Summary

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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 message delivery failures. The issue appears more likely when the sending account is an Outlook profile linked to another Exchange account or when an Exchange Online mail contact shares the same SMTP address. Microsoft says the problem is still under investigation and has not announced a fix yet. Temporary workarounds include removing the M365 account Address Book, creating a new classic profile, or sending mail through New Outlook or Outlook.com on the web.

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 Windows Autopatch fix for EU restricted driver update deployment bug

Security Tool/Service
First: 13.05.2026 17:36 Last: 13.05.2026 17:36 Sources 1

About this happening: **Microsoft** fixed a **Windows Autopatch** service bug that let **restricted driver updates** reach some managed devices in the **EU**, bypassing admin approval controls and crea...

Open Happening

Microsoft Windows 365 Office installation disruption

Service Disruption
First: 13.05.2026 14:53 Last: 13.05.2026 14:53 Sources 1

About this happening: The **Windows 365** service update has introduced a **configuration change** that is blocking **Office downloads and installs** for some customers, disrupting access on cloud PCs....

Open Happening

Microsoft Exchange Online blocks legacy TLS for POP3 and IMAP4 starting July 2026

Security Tool/Service
First: 28.04.2026 16:18 Last: 28.04.2026 16:18 Sources 1

About this happening: **Microsoft** will block **TLS 1.0** and **TLS 1.1** for **POP3/IMAP4** access to **Exchange Online** in **July 2026**, which could break legacy mail clients and embedded devices...

Open Happening

Timeline

  1. 06.04.2026 22:19 1 articles · 1mo ago

    Microsoft fixes Classic Outlook send failure via Outlook.com

    Mitigation Patch Update

    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.

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  2. 02.04.2026 12:12 1 articles · 1mo ago

    Microsoft investigates Classic Outlook email sending failures via Outlook.com

    Initial Disclosure

    Microsoft is investigating a known issue affecting some Classic Outlook users that can prevent sending or replying to email through Outlook.com and can return NDR errors such as 0x80070005-0x0004dc-0x000524. The failure appears more likely when the Outlook.com sending account is an Outlook profile linked to another Exchange account or when the sender has an Exchange Online mail contact with the same SMTP address, and Microsoft has recommended temporary mitigations such as removing the M365 account Address Book, creating a new classic Outlook profile, or using New Outlook or Outlook.com on the web.

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