Administrators running Exchange Server on Windows Server 2019 or later should not uninstall a recent Windows Server security update, even if Outlook has stopped working. Microsoft has a temporary workaround instead.
According to a Microsoft security note, the update has caused access problems for Outlook on some Exchange Server systems. Affected users may be unable to open Outlook, or messages they send may sit in the outbox and never leave. Microsoft said the severity varies from server to server, and some servers see no trouble at all. There is no permanent fix yet.
In a separate post, Microsoft told customers to keep the update installed and apply a manual change. On the Exchange server, open a Command Prompt with administrator rights, run the command below, then reboot:
reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /fThe command edits the Windows registry, the database of low-level system settings, under a key labeled Rpc. That abbreviation stands for remote procedure call, a way for programs to request services from one another across a network. The instructions do not explain why this particular value clears the problem.
The guidance not to roll back is the telling part. A security update exists to close holes, so pulling it trades a mail outage for renewed exposure. Microsoft is asking administrators to keep the patch and hand-edit a setting instead, which is a reasonable call but not a free one.
The burden also lands on a narrow group. Ordinary Outlook users cannot fix this themselves, and Exchange is software that organizations typically run on their own servers, so the repair depends on an IT team seeing the note and acting on it. Microsoft has not said when a long-term fix will arrive, which leaves the registry edit as the only official option for now.