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Fix Exchange 2016 CU23 to Microsoft 365 Connector Issues
Question details
The connection from an on-premises Exchange 2016 server to Microsoft 365 stopped functioning after installing Cumulative Update 23 (CU23), disrupting mailbox migrations.
- Product
- Microsoft Exchange Server 2016
- Device & OS
- not provided
- Scenario
- Conducting mailbox migrations in an Exchange hybrid environment.
- Observed behavior
- The on-premises to Microsoft 365 connector breaks and stops working immediately following the installation of Exchange 2016 CU23.
Before modifying your server configuration, ensure you have gathered all relevant connector error messages and application event logs from the Event Viewer to help identify the root cause.
Consult Exchange Server Management Specialists on Microsoft Q&A
Since this is a complex hybrid configuration issue caused by a specific Cumulative Update, getting tailored help from dedicated Exchange specialists is highly recommended.
Connector failures following a Cumulative Update often involve complex TLS certificate bindings, routing changes, or underlying OS security patches introduced by the CU. Because this involves advanced server architecture rather than a simple configuration toggle, it is best addressed by dedicated Exchange Server engineers.
Collect your hybrid configuration details, specific connector error codes, and relevant event logs from your Exchange 2016 server's Event Viewer (Application log).
Open your web browser and go to the official Microsoft Q&A community platform.
Select the 'Exchange Server Management' tag when creating your thread. Post your detailed question, explicitly mentioning the recent CU23 installation, to get targeted assistance.
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Frequently Asked Questions
Why did Exchange 2016 CU23 break my hybrid connector?
Cumulative Updates often introduce security patches or configuration resets that can inadvertently affect custom receive/send connectors, TLS certificates, or virtual directory settings required for Microsoft 365 hybrid communication.
Where can I find the connector error logs in Exchange 2016?
You can find connector error logs by opening the Event Viewer on your Exchange server, navigating to Windows Logs > Application, and filtering for the 'MSExchangeTransport' and 'MSExchangeFrontEndTransport' event sources.
Can I roll back the Exchange 2016 CU23 update?
Microsoft does not officially support uninstalling Cumulative Updates for Exchange Server. It is recommended to fix the configuration issue or restore the server from a full backup made prior to the update.
Does running the Hybrid Configuration Wizard (HCW) fix connector issues?
Yes, re-running the latest version of the Office 365 Hybrid Configuration Wizard can often repair connector configurations, bindings, and certificate trusts that were altered or reset by a Cumulative Update.
