Either a Windows Update or the Exchange SE update from yesterday broke Exchange SE. Official

This is just a warning incase you guys haven't seen it yet:

[https://techcommunity.microsoft.com/blog/exchange/released-october-2026-exchange-server-security-updates/4563014](https://techcommunity.microsoft.com/blog/exchange/released-october-2026-exchange-server-security-updates/4563014)

**Update October 15, 2026:** We have received reports indicating that installation of October 2026 updates for Windows Server - KB5126646 (Windows Server 2025), KB5126655 (Windows Server 2022) or KB5126651 (Windows Server 2019) - can cause Exchange Server OWA to fail with an error *MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)* with event ID 1006 logged in the Application Log, as well as growing email queues. Installation of October 2026 Exchange updates is *not required* for issues to occur. We are investigating this with our Windows partners.

The scariest part of this is the note: "Installation of October 2026 Exchange updates is *not required* for issues to occur. We are investigating this with our Windows partners." Good luck, have fun.

Rebooting the mailbox server in our case restored mail flow and OWA but I am guessing that is a temporary fix.

This is kind of a terrible issue because Outlook was acting like everything was perfectly normal and you didn't get the normal "user screaming at your because outlook doesnt work".