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
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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?
My previous comment is probably still pending approval, so I’m posting this again here: we are seeing the same issues in our environment, but we only installed the October Windows updates (KB5126646, KB5126645 and KB5127690) on Windows Server 2025, not the October Exchange SU (KB5128895). Our Exchange Server SE RTM DAG is still on September 2026 SU v2 (15.2.2562.53).
Thank you, this is very important information.
We have two affected Windows Server 2025 systems with exactly the same Windows updates installed:
- KB5126645
- KB5126646
- KB5127690
Both systems experienced MapiExceptionMdbOffline (ec=1142) approximately 24–36 hours after the updates.
Unlike your environment, we also installed Exchange SE October SU KB5128895.
Since your environment is still running Exchange SE build 15.2.2562.53, this strongly suggests that the Exchange October SU itself may not be required to trigger the issue.
Could you confirm whether you are seeing the same MapiExceptionMdbOffline error (ec=1142), with mailbox databases still showing as mounted?
This could help narrow down the root cause to the October Windows updates or an interaction with Exchange SE.
Yes, I can confirm. We see exactly the same error, and the databases were still showing as mounted and healthy at the time:
Event ID 1006 (MSExchange Mailbox Replication)
The Microsoft Exchange Mailbox Replication service was unable to process jobs in a mailbox database.
Database: DB01
Error: MapiExceptionMdbOffline: Unable to make connection to the server. (hr=0x80004005, ec=1142)
ECP was still working, and both ECP and the Exchange Management Shell showed the databases as mounted and healthy. However, we could not move them (switchover failed), and the Exchange services could not access them. OWA was down and the transport queues were filling up. Only a reboot resolved it.
The timing matches yours as well. The updates were installed during the night of October 13, and the problems started on the evening of October 14, roughly a day later.
So we have the same three Windows updates (KB5126645, KB5126646, KB5127690) and the same error, with and without KB5128895. That points pretty clearly to the October Windows updates, or an interaction between them and Exchange SE, rather than the Exchange October SU itself.