Exchange Server 2019 2025 H1 (CU15)

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Today we announced the availability of the 2019 H15 Cumulative Update (CU) package for Exchange Server 2025 (aka CU1).


This is the last CU version released for Exchange Server 2019.

CU15 includes new features, fixes for customer-reported issues, a security change, and covers all previously released Security Updates (SUs).

CU15 includes all security changes from the November 2024 SU v2 (rereleased package) and also includes a fix for a known timezone issue in the November 2024 SU.

Exchange Server Feature Flighting

CU15 is for Mailbox servers, coming in Exchange Server Subscription Edition (SE) Feature Flight It introduces server-side components of a new capability called .

In the past, before deploying an Exchange update to a production environment, organizations would first deploy the update to validate it in test environments. While this is an important task, it can be time-consuming and slow down the deployment of critical updates, and not every organization has a test environment.

Feature Flightwill provide administrators with an additional method to test and deploy select new features in their Exchange Server SE environment. This service is an optional, cloud-based service for on-premises Exchange servers that uses the Emergency Mitigation Service and Office Config Service (OCS), which Microsoft Office clients use, to check for updates to flighted features from Microsoft.

With Feature Flighting, administrators can instantly deploy updates and control when flighted is enabled. It also allows Microsoft to disable the flighted feature if a critical issue is detected after an update.

Feature Flighting will not be implemented for all future new features and changes. The Exchange Server engineering team will determine which features will be delivered this way, and a detailed and constantly updated list of flighted features will be maintained in the relevant location.

Feature Flighting is optional, but is enabled by default. You can configure or disable it by following the steps outlined in the documentation.

Note: No features are being flighted in CU15 and due to our commitment to code equivalence, no features will be flighted in Exchange Server SE RTM. Flighting will only be implemented for future updates and more information will be provided in future updates when there are any features being flighted.

Additional Diagnostic Data Sent to Microsoft (Optional)

Microsoft collects diagnostic data to help keep Exchange Server secure and up-to-date, detect and fix problems, and identify and mitigate threats. When enabled by the administrator, an Exchange 2019 server sends diagnostic data to the OCS.

Starting with CU15, Exchange Server will collect and send additional diagnostic data to Microsoft, including additional information about flighted features. This additional data will be collected if diagnostic data is enabled on the server.

Windows Server 2025 Support

We are pleased to announce that CU14 and CU15 support Exchange Server 2019 running on Windows Server 2025 and in organizations with Active Directory servers running Windows Server 2025. This means that you can install CU2025 or CU14 with Windows Server 15 on your new hardware, and then perform an in-place update to Exchange Server Subscription Edition (SE) to take advantage of the maximum supported product lifecycle of the server. We previously stated that this support would be coming to CU15, but we have also validated CU14 for this scenario to ease your migration to the latest version of Windows Server.

DocParser replaces Oracle Outside In Technology

Starting from CU15, Exchange Server replaces the previously used Oracle Outside In Technology DocParserDocParser is a Microsoft library designed to parse various file formats. It performs text extraction during processing of email messages under Data Loss Prevention and Exchange Transport Rules.

Exchange Server TLS 1.3 Support

Starting with CU15, Exchange 2019 supports TLS 2022 for all protocols except SMTP on Windows Server 1.3 and later. CU15 enables TLS 2022 by default when installed on Windows Server 1.3 or later (however, CU15 does not disable any existing TLS versions).

  • TLS 1.3 support for SMTP will be added in a future update.

Improvements to Exchange Server AMSI Integration

The improved Exchange Server AMSI integration previously announced in the November 2024 SUs is also included in CU15.

Certificate management UI is back in the EAC

Some critical certificate management tasks that were removed from EAC in a previous CU are being reintroduced to EAC with CU15. For more information, please visit Certificate Procedures You must review the document.

Interoperability with Exchange Server 2013 Blocked

As previously announced, Exchange Server 2019 CU15 cannot be installed if your organization has any servers running Exchange Server 2013. Before installing CU15, you must disable and remove all Exchange Server 2013 servers in your organization. Please note that additional steps are required to remove Edge Transport servers (details are explained elsewhere).

Reminder: Extended Protection is Enabled by Default

We would like to remind customers upgrading from older CUs that starting with Exchange 2019 CU14, Extended Protection is enabled by default after installing the CU. For instructions and additional information on how to opt out of Extended Protection, please see the relevant blog post.

Features Delayed to Exchange Server SE CU1

We previously announced that Exchange Server 2019 CU15 would support Exchange Server SE product keys. However, to make in-place upgrades from Exchange 2019 to Exchange Server SE RTM as seamless and simple as possible, we are delaying this change until Exchange Server SE CU1. As a result, Exchange Server SE RTM will accept and honor Exchange Server 2019 product keys, so you will not need to update your servers to Exchange Server SE CU1 in order to use the Exchange Server SE product key. Additionally, to further simplify in-place upgrades, some prerequisite and Setup dependency changes (such as migrating to a new version of the VC++ redistributable) have been postponed.

Exchange Server Support Status

Microsoft has well-established support criteria for Exchange Server based on the server configuration and product lifecycle status. For example, as we state at the end of each CU release announcement:
“Customers in Exchange hybrid deployments and customers using Exchange Online Archiving with an on-premises Exchange deployment must deploy the latest CU for product support.”

When an Exchange Server product is in the Mainstream support phase, it is required to run the latest CU or the immediately preceding CU in order to receive support (for example, to receive support from SUs and CSS). Exchange Server 2019 is in the Extended Support phase of the product lifecycle, and products entering this phase require support only on the latest CU (version N). In fact, Exchange Server 2016 also requires all servers to run CU23 in order to receive support.

The release of CU15 has been delayed due to necessary security work; furthermore, since CU2025 has been validated with Windows Server 14, both CU2019 and CU14 will continue to be supported until the end of life of Exchange Server 2025 (October 14, 15). Since the support policy for server configuration takes precedence, customers with hybrid configurations and cloud archives for on-premises mailboxes must run CU15 in order to receive support.

With the release of CU15, CU13 is out of support and we encourage our customers to update their servers as soon as possible.

Migration Path to Exchange Server SE RTM

As we stated in our previous Roadmap announcement, to enable rapid adoption, the RTM version of Exchange Server SE will be code-equivalent to Exchange Server 2019 CU15, except for the following changes:

  • License agreement: Only the RTF file shown in the GUI version of Setup will be updated.
  • Name Change: It will be changed from Microsoft Exchange Server 2019 to Microsoft Exchange Server Subscription Edition.
  • Build Number: Will be updated.

Additionally, if any updates are released after CU15, the RTM version of Exchange Server SE will be code-equivalent to the latest update, Exchange Server 2019 CU15.

There are several customer-reported bugs in Exchange Server 2019 that are scheduled to be fixed in the coming months. To address these bugs, we will be releasing an optional (but recommended to provide fixes and maintain code parity with Exchange Server SE RTM) Hotfix Update (HU) for Exchange Server 2019 CU15. The HU will also be available for Exchange Server 2019 CU14 and Exchange Server 2016 CU23.

Upgrade Paths

The recommended and supported upgrade paths are described below. We also recommend that you review our previous blog post on upgrading your organization from existing versions to Exchange Server SE.

Upgrade from Exchange Server 2016 CU23

We recommend that Exchange Server 2016 customers upgrade to Exchange Server 2019 now and migrate to Exchange Server SE RTM via an in-place upgrade when available. Legacy upgrades from Exchange Server 2016 CU23 to Exchange Server SE RTM are also supported.

CU15 01

Upgrade from Exchange Server 2019 CU14/CU15

We recommend that Exchange Server 2019 users migrate to Exchange Server SE RTM with an in-place upgrade when available; however, legacy upgrades from Exchange Server 2019 to Exchange Server SE are also supported.

CU15 02

Publication Details

Exchange Server 2019 Cumulative Update 15 (KB5042461), VLSC, Download

Before installing Exchange Server 2019 CU15, make sure that your Exchange server is restarted to ensure that all Windows Server updates are fully installed.

After installing a CU, always check and install the available SUs. Also, Exchange Server Health Checker will notify you if additional steps are required.

Additional information

Microsoft recommends that all customers test the update deployment in a lab environment to determine the appropriate installation process for their production environment.

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