Windows Server November 2026 Patch Tuesday Updates Fix Exchange Server Sign-In and Email Delivery Issues

Microsoft has finally fixed one of the annoying Exchange Server issues caused by October 2026 Windows Server updates. The November Patch Tuesday updates, KB5131080 and KB5131090, resolve Outlook sign-in and email delivery failures. Administrators can now install the latest cumulative updates instead of relying on Microsoft’s temporary registry workaround.

November 2026 updates fix Exchange sign-in failures

Microsoft confirmed the fix via its Windows release health dashboard. The company recommends installing the November 2026 updates on all affected Exchange Server systems. Per the company, the fixes address issues affecting on-premises Exchange environments. These include users being unable to sign in to Outlook on the web (OWA). Some outgoing messages also remained stuck in Outlook’s Outbox instead of reaching recipients.

The November 2026 Patch Tumuesday updates also address issues accessing mailboxes during mailbox replication. This should help administrators restore normal email operations without depending on temporary workarounds. The fix is especially important for organizations that postponed updates while waiting for Microsoft’s resolution. Those administrators can now move forward with the November security updates.

What triggered the Exchange Server issues in October?

The issues stemmed from October 2026 Windows Server security updates. The affected systems included Windows Server 2016, 2019, 2022, and 2025. Ar the time, Microsoft explained that October 2026 Patch Tuesdsy updates introduced a new default memory limit for Endpoint Mapper. This Windows component handles Endpoint Lookup calls, and the restriction could eventually disrupt Exchange communications.

As a result, affected organizations experienced Outlook sign-in failures, delayed email delivery, and mailbox replication problems. The severity varied depending on individual server workloads.

The affected Windows updates included KB5126646, KB5126655, KB5126651, and KB5126868. Microsoft previously recommended a registry workaround involving the EpmapSavedContextCap value. Now that the fix is available, Microsoft recommends installing the November 2026 Windows Server updates. Organizations that previously created the registry workaround can leave it in place. It is no longer required after installing the November updates.