Microsoft has released Hotfix Update (HU) for:
- Exchange Server Subscription Edition (SE)
HU is available for the following specific version of Exchange Server:
The May 2026 HU does not contain any new Exchange Server security updates but contains new functionality. Please see the release KB article for more information.
Updating your Exchange rich hybrid coexistence to Graph API calls
May 2026 hotfix update contains functionality that will allow you to start switching your Exchange Server hybrid rich coexistence from using Exchange Web Services (EWS) to REST-based Microsoft Graph API calls. This is a continuation of work we announced in Exchange Server Security Changes for Hybrid Deployments.
Note: Exchange 2016 and 2019 are out of support. We are not releasing an update for those versions to use Graph API hybrid calls (even updates released under Exchange 2016 or 2019 ESU will not contain this functionality). Customers who are still using Exchange 2016 or 2019 servers to host mailboxes on premises will have to keep allowing EWS use in their tenant past October 2026 and must upgrade all servers to Exchange SE by April 2027 (when EWS is disabled in Exchange Online). Rich coexistence features on those unsupported versions will permanently stop working in April 2027. Please upgrade your on-premises environments from unsupported versions ASAP. By running unsupported versions, you might be putting your environment at risk!
More information can be found in Update Your Exchange SE Hybrid On-premises Rich Coexistence to Graph and Deploy dedicated Exchange hybrid app.
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.
- After setup is completed, please reboot the server and check that all Exchange services have started properly. If some services are in a disabled state, that indicates that something interrupted installation of the update. Please see the Workaround 1 in this article.
- 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
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. Exchange Hotfix Updates are optional.
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?
This HU is an optional update for your servers and the update will be shown as an optional update on Windows / Microsoft update a few days after general release on Download Center.
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 SE.
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.
Updates to this post:
- 5/8/2026: Updated the FAQ related to Windows / Microsoft Update availability
The Exchange Server Team
