Microsoft has released Hotfix Updates (HUs) that address issues in earlier updates for:
- Exchange Server Subscription Edition (SE)
- Exchange Server 2019
- Exchange Server 2016
HUs are available for the following specific versions of Exchange Server:
The September 2025 HUs do not contain any new Exchange Server security updates but contain a fix for a non-security issue. Please see the release KB article for more information.
Reminder: Dedicated Exchange Hybrid Application support
We wanted to remind you that since Exchange updates are cumulative, September 2025 HUs include support for creation of the dedicated Exchange hybrid app originally announced during April 2025 HU release. Please see our post Dedicated Hybrid App: temporary enforcements, new HCW and possible hybrid functionality disruptions to evaluate action appropriate for your organization. Check the feature documentation (including the FAQ).
Update installation
The following update paths are available:
- Inventory your Exchange Servers to determine which updates are needed using the Exchange Server Health Checker script. Running this script will tell you if any of your Exchange Servers are behind on updates (CUs, SUs, or manual actions).
- Install the latest CU. Use the Exchange Update Wizard to choose your current CU and your target CU to get directions.
- Re-run the Health Checker after you install an update to see if any further actions are needed.
- If you encounter errors during or after installation of Exchange Server, run the SetupAssist script. If something does not work properly after updates, see Fix failed Exchange Server updates. Also please see File version error when you try to install Exchange Server updates.
Hotfix Update FAQs
The timing of this release seems a bit strange; why did Microsoft decide to release this HU at the start of the month? Is this urgent?
Hotfix releases are not tied to the “patch Tuesday” release schedule as they do not contain security updates. We became aware of a scenario that some of our hybrid customers ran into when deploying the dedicated Exchange hybrid app and wanted to release a fix as soon as possible so customers who are impacted can solve it. Please see the release KB for more information.
We installed the last Security Update. Should we install the later Hotfix Update?
Exchange Server HUs are optional updates, but they might introduce features or fixes that your organization can benefit from. Please see the release KB article for more details.
We did not yet install the earlier Security Update. Do we have to install the last available SU first before installing the later HU?
All of Exchange updates (HUs or SUs) are cumulative. Therefore, a newer SU or HU will contain all the changes that a previous, older SU or HU has. If you have not installed the older updates yet, you can install the newer one directly and skip the older one.
Our Exchange servers update automatically through Windows / Microsoft Update. Will our servers automatically install the HU update?
The HU is available through Windows / Microsoft Update. Installation will follow your administrator's Windows Update deployment policy.
Will the new features and fixes released in the HU also be rolled into future updates, or must we install this specific HU to get them?
Content of this HU will be included in subsequent updates for Exchange Server.
Can HUs be uninstalled (if the need arises)?
Yes. HUs, like SUs, can be uninstalled.
Documentation may not be fully available at the time this post is published.
This post might receive future updates; they will be listed here (if available).
The Exchange Server Team
