Almost a month ago, Microsoft released Security Updates (SUs) for multiple versions of Exchange Server. Now, the company has released Hotfix Updates (HUs) for three supported editions of Exchange Server too. It is important to note that these are not tied to the Patch Tuesday release cadence.
The latest HUs do not contain any security patches but they do fix a major issue related to online archival processes failing for on-premises users in hybrid environments. This was impacting customers who had items configured with the Move to Archive tag, with their items not being archived if the archive is present in Exchange Online and the primary mailbox remains on an on-premises server that"s running Exchange Server.
The problem was impacting affecting the following versions of Exchange Server, all of which have been patched through the latest HUs:
- Exchange Server Subscription Edition (SE) RTM
- Exchange Server 2019 (CU14 and CU15)
- Exchange Server 2016 (CU23)
If you are running any of the aforementioned CUs, you can directly installed the September 2025 HUs and stay up to date. But if you"re on an older version, you"ll first need to install a supported CU and then apply the HU. Since Exchange updates are cumulative, the September 2025 HUs notably include support for creation of the dedicated Exchange hybrid app that was initially present in April 2025 HUs.
Also note that HUs are typically optional updates so if you"re impacted by the online archival problem explained previously, you should necessarily install the update, but if you"re not, you can consider skipping it. HUs can be downloaded through Windows Update and Microsoft Update, but their installation will follow the deployment policies configured by your IT admin. Even if you decided to skip this HU, the improvements in this release will be present in subsequent HUs, SUs, and CUs, but if you do install it and run into unexpected errors, you can simply uninstall it.