[German]Ahead of the September 2025 Patch Tuesday on September 8, 2025, Microsoft has released a hotfix update for the on-premises Exchange Server versions still under support (Exchange Server Subscription Edition (SE), Exchange Server 2019, Exchange Server 2016). Hotfix versions are not tied to the Patch Tuesday release schedule because they do not contain security updates, only functional bug fixes. Currently, an archiving issue in hybrid environments is being fixed.
I became aware of the hotfix updates for Exchange Server via the following tweet and the article by my colleagues. Microsoft's Exchange team published the Tech Community article Released: September 2025 Exchange Server Hotfix Updates.
According to a Techcommunity article, hotfix updates are available to fix the problem described below for the following Exchange items.
- Exchange Server Subscription Edition (SE) RTM
- Exchange Server 2019 CU14, and CU15
- Exchange Server 2016 CU23
It should be noted that Exchange Server 2016 and Exchange Server 2019 will no longer be supported as of October 14, 2025, and will no longer receive updates. Exchange Server Subscription Edition (SE) will then be the only on-premises Exchange Server version still supported. After installing the hotfix update, the Exchange Health Checker should be run to verify that the installation is OK
Hotfix patches an archiving problem
The hotfix updates resolve an issue that occurs after activating the dedicated Exchange Hybrid Application. According to the Microsoft Support article Online archiving fails for on-premises users in hybrid environment, items configured with Move to Archive are no longer automatically moved to the archive mailbox if the archive is located in Exchange Online and the primary mailbox remains on a local Microsoft Exchange Server.
Dedicated Hybrid App Enforcement Delays
Microsoft referred to the dedicated hybrid app in April 2025 in the article Dedicated Hybrid App: temporary enforcements, new HCW and possible hybrid functionality disruptions. The original version stated that Microsoft wanted to start temporarily blocking Exchange Web Services (EWS) traffic in August 2025 using the Exchange Online Shared Service Principal (which is used by default by some coexistence features in hybrid scenarios).
This was part of a phased strategy to accelerate the adoption of the dedicated Exchange hybrid app by our customers and make our customers' environments more secure, it said. However, the post was updated on August 18, 2025. Based on customer feedback, this enforcement mode will not come into effect until September 2025.
