Microsoft announced CU15 or 2025 H1, the last cumulative update for Exchange Server 2019. In addition to bug fixes, it brings a number of new features, including the Fligthing feature for the gradual introduction of new functions. It also runs in environments with DCs under Windows Server 2025 and brings back certificate management in the EAC.
As part of the Exchange roadmap, CU15 is not only the last cumulative update for Exchange 2019, but it also represents the bridge to the Exchange Subscription Edition (SE). Its RTM release is said to be based on the exact same code as CU15, only the name and build number change.
For the migration to Exchange SE, Microsoft had announced that CU15 would already accept the license keys for SE. Instead, Exchange SE will support the product keys of version 2019. Separate SE keys will only be available with Exchange SE CU1.
Feature Flighting
One of the most important innovations in CU15 is the so-called feature flighting . This controls the installation of new features via the cloud service Office Config Service . It is intended to ensure that major innovations are introduced into productive environments gradually and in a controlled manner.
New features are installed by default, but are only activated depending on the configured settings. The distribution rings that admins can assign to their Exchange servers are crucial for this. These have the following meaning:
- Early Adopter Ring (0): Exchange servers in this ring are the first to be deployed; it is used to test new features, which become active immediately after installation;
- Worldwide Ring (1): This is the default ring that Exchange servers are placed in after installing CU15. New features are activated there as soon as Microsoft declares them suitable for general use;
- Admin Action Ring (2): Exchange servers in this ring do not activate any new features. In addition, it ensures that previously activated features are deactivated again.
Microsoft obviously wants to create a safety net to mitigate the consequences of faulty updates. This way, the manufacturer can globally recall a new feature if it leads to serious disruptions in productive environments.
In practice, the effect of feature flighting will remain limited for some time. Currently, there is only one new feature on the list called Heartbeat Probe . In addition, not all new features will be designed for flighting in the future.
Process of installing new Exchange features with flighting
Further innovations
In addition to the Feature Flighting, the CU15 brings a number of other improvements:
- Support for Windows Server 2025: The compatibility with Active Directory in Windows Server 2025, originally intended only for CU15, now also applies to CU14. To maximize the product lifecycle, Microsoft recommends installing Exchange CU14 / CU15 on new hardware with Windows Server 2025 and later migrating to Exchange Server SE using an in-place upgrade.
- A previous CU removed some important certificate management tasks from the Exchange Admin Center GUI. These are returning with CU15.
- Starting with CU15, Exchange 2019 supports TLS 1.3 on Windows Server 2022 and higher. However, SMTP is an exception;
- Transfer of additional diagnostic data: With CU15, the list of data transferred to Microsoft is significantly longer (assuming this option is enabled). In addition to data for monitoring feature flighting, this also includes data that has no obvious technical use. This applies, for example, to the number of mailboxes, the list of accepted domains or a device ID.
Longer support for CU14
Exchange Server 2019 is already in extended support, meaning the N-1 rule of the first five years no longer applies. This means that the manufacturer would normally only support the latest release, i.e. CU15.
Due to the delay for CU15 and the now validated compatibility of CU14 with Windows Server 2025, it remains at N-1 and CU14, like CU15, will be supported until October 14, 2025.
An exception are hybrid environments and those that archive their on-prem mailboxes in the cloud. They must be on the latest CU.
Conclusion
At first glance, CU15 brings some interesting new features. However, the practical use of feature flighting will remain very limited until Exchange SE CU1 is released. For AD environments with Windows Server 2025, CU14 is also sufficient, and will also be supported until the end of Exchange 2019. It will therefore also receive the hotfix updates that are still planned.
The recommended in-place upgrade to Exchange Server SE also works with CU14, so an upgrade to CU15 does not promise any particular benefit if Exchange 2019 is already at CU14 level – unless companies consider the partial support for TLS 1.3 and the restored certificate management in the EAC to be sufficient for the effort involved.