[German]Microsoft has released the 2025 H1 Cumulative Update for Exchange Server 2019 on February 10, 2024. The CU15 is the last cumulative update for this Exchange version, which will then be withdrawn from support in October 2025. The CU15 contains all security changes that are included in the SU v2 from November 2024, as well as a correction for the known time zone problem in the SU from November 2024 (see also Microsoft Exchange Server Nov. Updates Re-Release (27. Nov. 2024)).
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I became aware of the release via the following tweet. Microsoft has published the Techcommunity article Released: 2025 H1 Cumulative Update for Exchange Server.
CU15 is also referred to as 2025 H1 for Exchange Server 2019 and is the last CU that Microsoft will release for Exchange Server 2019. CU15 contains new features, fixes for customer-reported issues, a security change and integrates all previously released security updates (SUs).
Exchange Server Feature Flighting
In the past, organizations often had to deploy updates in a test environment before installing an Exchange update in the production environment. The aim was to validate the update before deploying it to the production environment.
CU15 introduces the server-side components for a new feature for mailbox servers called Feature Flighting. The feature will be available with the Exchange Server Subscription Edition (SE) (not yet available in CU15). Feature Flighting is intended to offer administrators an additional opportunity to test and introduce selected new functions in an Exchange Server SE organization.
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Feature Flighting is an optional cloud-based service for on-premises Exchange servers that will apply to future updates. The feature uses the Office Config Service (OCS) – the same endpoint used by the Emergency Mitigation Service and Microsoft Office clients – to check for updates from Microsoft related to these features.
Feature Flighting allows administrators to deploy updates immediately and control when a feature is activated in their environment. Feature Flighting also allows Microsoft to disable a rolled out feature if a significant issue is discovered after the update is released.
Feature Flighting will not apply to all new features and changes in future updates. The Exchange Server development team will determine which features will be distributed via Feature Flighting and will also maintain an up-to-date list of the features concerned. The use of Feature Flighting is optional, but is enabled by default.
Optional additional diagnostic data
According to the announcement, Microsoft collects additional diagnostic data after CU15 is installed and sends it to Redmond to keep Exchange Server secure and up to date, find and fix problems, and identify and mitigate threats. When enabled by an administrator, an Exchange 2019 server will send diagnostic data to the OCS.
Support for Windows Server 2025
Both CU14 and CU15 provide support for running Exchange Server 2019 on Windows Server 2025 (as well as for organizations using Active Directory servers running Windows Server 2025). This means that CU14 or CU15 can be installed on new hardware running Windows Server 2025 and can then be upgraded to Exchange Server Subscription Edition (SE).
DocParser replaces Oracle Outside In technology
As of CU15, Exchange Server uses DocParser as a replacement for the Oracle Outside In technology previously used in Exchange Server. DocParser is a Microsoft library that was developed for parsing various file formats. It performs text extraction when processing emails in transport for Data Loss Prevention and Exchange Transport Rules.
TLS 1.3 partial support
Starting with CU15, Exchange 2019 supports TLS 1.3 on Windows Server 2022 and higher for all protocols except SMTP. When CU15 is installed on Windows Server 2022 or later, TLS 1.3 is enabled by default (note, however, that CU15 does not disable any existing version of TLS). Support for TLS 1.3 for SMTP will be added in a future update.
Further improvements / changes
The improved Exchange Server AMSI integration (already announced in the November 2024 SUs) is also included in CU15.
Some important certificate management tasks were removed from the EAC in a previous CU. CU15 reintroduces certificate management in the EAC.
Exchange Server 2019 CU15 cannot be installed in an organization that still runs Exchange 2013 servers. All Exchange 2013 servers in the organization must be decommissioned and uninstalled before CU15 can be installed.
Microsoft would also like to remind you that Extended Protection has been activated by default since CU14.
Delay for Exchange Server SE CU1
Microsoft had announced that Exchange Server 2019 CU15 would provide support for Exchange Server SE product keys. This is not the case, Microsoft are delaying this change until Exchange Server SE CU1 is available. This is to ensure that upgrades from Exchange 2019 to Exchange Server SE RTM are as smooth and easy as possible.
Therefore, Exchange Server SE RTM will accept Exchange 2019 product keys. Administrators do not need to upgrade their servers to use an Exchange Server SE product key up to Exchange Server SE CU1. To further simplify in-place upgrades, Microsoft has also postponed some changes to prerequisites and setup dependencies (e.g. switching to a new version of VC++ Redistributable). Further details can be found in the linked Techcommunity article.
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