By Guru Baran
Publication Date: 2026-10-19 03:39:00
Microsoft has confirmed that Windows Server security updates released on October 13, 2026, can disrupt Exchange Server email services, blocking webmail access and leaving messages undelivered.
The issue affects some on-premises systems running Exchange Server 2016, Exchange Server 2019, and Exchange Server Subscription Edition. Microsoft has published a workaround and advises administrators not to uninstall the Windows updates.
The failures stem from changes in Windows Server, not the separate October Exchange Server security updates. Servers can experience problems even without installing the latest Exchange patches, making the underlying operating system update the key factor when investigating these outages.
Affected Updates and Exchange Server Symptoms
Microsoft identified four Windows updates linked to the failures. These are KB5126646 for Windows Server 2025, KB5126655 for Windows Server 2022, KB5126651 for Windows Server 2019, and KB5126868 for Windows Server…
