October's Windows Server update left Exchange mail stuck in the Outbox. No fix yet.
By Lior AharonovFounder, 7IT Solutions ·
Microsoft says Windows Server 2019 and later, after the October 2026 security updates, can break Exchange: Outlook on the web sign-in fails, mail stays in the Outbox and mailbox replication struggles. The update capped Endpoint Mapper lookups at 512 simultaneous calls, which may be too few under heavy load. A fix is in progress; Microsoft advises against uninstalling the updates.
Your sales team hits send on the quote. It sits in the Outbox. Nobody sees an error until the customer asks where it went, and the busiest servers are the most exposed.
If you run Exchange on Windows Server 2019 or later patched in October, apply Microsoft's EpmapSavedContextCap registry workaround from an administrator command prompt, reboot, and keep the security updates installed.
Security patches change limits nobody wrote down. Watch your mail queue on patch week the way you watch a deploy.
Lior Aharonov · my takeSources
- Microsoft: October Windows Server updates cause Exchange issues (BleepingComputer) bleepingcomputer.com
Researched with AI tools; every fact is checked against the linked sources.