Microsoft has fixed the Outlook sign-in and stuck-email problems that hit some company Exchange servers after October's Windows Server updates.
Microsoft has released a fix for a problem that left some businesses unable to sign in to Outlook on the web or send email. The trouble started after the October 2026 Windows Server security updates. The fix comes in the November 2026 updates, KB5131080 and KB5131090.
Microsoft confirmed the problem in mid-October. It affected on-premises Exchange servers, meaning email servers a company runs itself, on Windows Server 2019 and later. Some users could not sign in to Outlook on the web. Others found that messages sent from Outlook sat in the Outbox and never left.
Microsoft traced it to a new default limit introduced by the October updates. The limit caps how much memory a Windows component called Endpoint Mapper can use when it handles Endpoint Lookup calls. Exchange could also struggle to reach mailboxes during mailbox replication, which is the copying of mailbox data between servers.
Not every server was hit equally. Microsoft said the severity and speed of the problem depended on how busy the Exchange server was, and some servers might never have seen it. The material names only companies running their own Exchange servers. It does not name people who use Microsoft 365 email.
Microsoft urged customers not to uninstall the October updates. Those are security updates, so removing them would trade one problem for another. The company also said the Outlook on the web issue is not linked to the October Exchange Server security updates.
The alert box above lists the steps. In short, find out whether your company runs its own Exchange server, then make sure the November updates are installed on every affected server.
Companies that cannot update right away have a stopgap. Microsoft shared a temporary workaround that an IT administrator applies by changing a registry setting, which is a stored configuration value in Windows. It raises the number of lookup calls and the memory Endpoint Mapper can use. The server then needs a restart.
After the restart, administrators should keep watching the Exchange environment. If the problems come back, Microsoft says to raise the Endpoint Mapper memory limit further. The workaround is meant to be temporary, so the November updates are the real fix.
If your company runs its own Exchange server, install the November updates, KB5131080 and KB5131090, and keep the October updates in place.
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