Final cumulative update for Exchange Server 2019 lands at last

Administrators rejoice! The 2025 H1 Cumulative Update (aka CU15) for Exchange Server 2019 has finally arrived, marking the end of an era for the server application.

CU15 is the final CU Microsoft will release for Exchange Server 2019, and it is running months late, a concern for some admins as the update is needed to help organizations with the transition. Lest we forget, Exchange Server 2019 reaches its end of life on October 14, 2025, so time was running out for the update.

Microsoft attributed the delay to "necessary security work." It said that since it could validate CU14 with Windows Server 2025, it would support CU14 and CU15 until the server reaches the end-of-life date.

Microsoft added: "Note that the support policy regarding server configuration takes precedence, so hybrid configurations and customers with cloud archives for on-premises mailboxes must run CU15 to be supported."

The release means CU13 is out of support, and coexistence with Exchange Server 2013 has been blocked. Microsoft said, "You must decommission and uninstall all Exchange 2013 servers from your organization before you can install CU15."

As well as a raft of fixes, CU15 includes Exchange Server Feature Flighting, an on-by-default feature to allow admins to immediately deploy some updates and control when the flighted update is enabled.

The feature is due in Exchange Server SE but has been added to the server-side components in CU15.

Microsoft said, "Historically, before installing an Exchange update in production, organizations often deploy updates in a test environment to first validate the update before deploying it to their production environment. This is an important task, but it is also time-consuming, and it can slow down the deployment of important updates. Moreover, not all organizations have test environments."

Considering the company's past form with updates, we're not sure that something ostensibly designed to dodge deployment in a test environment before hitting production is such a good idea. Testing first might slow down the deployment of important fixes, however, it is a prudent thing to do where possible.

The feature also allows Microsoft to turn off a flighted update if something is found to be amiss with it.

Microsoft is adding the acquisition of additional diagnostic data from Exchange Server, although administrators can turn it off if desired.

Other updates include partial TLS 1.3 support (TLS 1.3 support for SMTP is due in a future update.)

While there will likely be hotfixes before the release of Exchange Server SE, CU15 marks the end of an era for the old warhorse and the beginning of a busy few months for administrators. ®