Update October 16, 2026: For more information related to installation of October 2026 updates for Windows Server and the possible impact on Exchange Server, please see Exchange servers may become uns...
Updated Oct 16, 2026
Version 7.0
Blog Post
We are experiencing the same issue on three independent Exchange Server SE environments after installing KB5128895 (Build 15.2.2562.56).
After approximately 24–36 hours, mailbox access fails with MapiExceptionMdbOffline (ec=1142), although all databases remain mounted and the Exchange services are running.
Outlook and OWA stop working, and mail delivery queues enter Retry state.
A full server reboot temporarily resolves the issue.
The affected systems run Windows Server 2019 and 2025.
Has the Exchange Team acknowledged this issue or identified a workaround?
Did you install the October Windows and .NET updates at the same time?
Windows Server 2019, Updates were installed with WSUS
- Security Update For Exchange Server Subscription Edition (KB5128895)
- 2026-10 Cumulative Update for .NET Framework 3.5, 4.7.2 and 4.8 for Windows Server 2019 for x64 (KB5128923)
- 2026-10 Cumulative Update for Windows Server 2019 for x64-based Systems (KB5126651)
Thanks for your reply.
We compared all three affected Exchange Server SE environments.
All three servers are running Exchange SE with October SU KB5128895 (Build 15.2.2562.56) and experienced the same MapiExceptionMdbOffline issue approximately 24–36 hours after the updates.
The installed Windows updates and .NET Framework versions differ:
Server A:
- KB5126651, KB5126650, KB5127685
- .NET Framework 4.8, Release 528049
Server B and Server C:
- KB5126645, KB5126646, KB5127690
- .NET Framework 4.8, Release 533509
On Server C, the Exchange security update was installed first, followed by a reboot. The Windows updates were installed afterwards.
All three servers recovered after a full reboot.
The only identified update common to all three environments is Exchange KB5128895.
This suggests a possible regression in the Exchange security update, although an interaction with the October Windows updates cannot yet be ruled out.
For now, we are still on the September updates, with the October update for Exchange, and the issue does not seem to have occurred yet.
Exchange SE (october security update) on Windows Server 2025 (.Net 4.8.1 september security update).This may be related to the .NET update.
In my case: Yes
For now, we are still on the September updates, with the October update for Exchange, and the issue does not seem to have occurred yet.
Exchange SE (october security update) on Windows Server 2025 (.Net 4.8.1 september security update).
