Microsoft Fixes Outlook Sign-In Issues
November Patch Tuesday updates resolve mail delivery problems caused by October Windows Server security updates
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Microsoft fixes Outlook zero-day spoofing vulnerability, CVE-2026-124194, which is already under attack.
Story published. November Patch Tuesday updates resolve mail delivery problems caused by October Windows Server security updates
Microsoft has fixed Outlook sign-in and mail delivery issues on Exchange Server systems caused by the October 2026 Windows Server security updates. The issues, which affected on-premises Exchange servers running Windows Server 2019 and later, have been resolved with the November 2026 Patch Tuesday updates.
The problems were caused by a new default limit on the amount of memory Endpoint Mapper uses when handling Endpoint Lookup calls, introduced by the October 2026 updates. This limit led to users experiencing trouble accessing email or signing in to Outlook after installing the latest updates.
The November 2026 Patch Tuesday updates, specifically KB5131080 and KB5131090, address the known issue. Microsoft has advised affected users to install these updates on all affected systems. A temporary workaround is also available for those who cannot immediately install the updates. This workaround involves modifying a registry key on affected Exchange Server systems by running a specific command from an elevated Command Prompt.
## What happened The issues were first acknowledged by Microsoft in mid-October 2026, with the company confirming that users had trouble accessing email or signing in to Outlook after installing the latest updates. The sign-in and mail flow issues affected not only Outlook but also Exchange's ability to access mailboxes during mailbox replication.
## Why it matters The fix is important for organizations that rely on Exchange Server systems for email communication. The issues caused by the October 2026 updates had the potential to disrupt business operations, making it essential for Microsoft to provide a solution quickly. With the November 2026 Patch Tuesday updates, affected users can now resolve the issues and ensure uninterrupted email services.
The temporary workaround, which requires running the command 'reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f' from an elevated Command Prompt on the Exchange Server, provides an alternative solution for those who cannot immediately install the updates. However, Microsoft recommends installing the November 2026 Patch Tuesday updates as the permanent fix.
