The Exchange product group released the September 2025 Hotfix Updates for Exchange Server SE, Exchange Server 2019, and Exchange Server 2016.

Hotfix updates do not contain security fixes, but address issues. They also might introduce or add support for functionality changes, such as dedicated Exchange hybrid app support.

Exchange	SU/HU	Download	Build	KB	Supersedes
Exchange SE	2	Download	15.2.2562.27	KB5066373
Exchange 2019 CU15	4	Download	15.2.1748.37	KB5066372	KB5057651
Exchange 2019 CU14	7	Download	15.2.1544.34	KB5066371	KB5057652
Exchange 2016 CU23	18	Download	15.1.2507.59	KB5066370	KB5057653

Changes

The issue addressed in these hotfixes is:

• Online archiving fails for on-premises users in hybrid environment

Dedicated Exchange Hybrid Application

A gentle reminder that since the April 2025 security updates, Exchange hybrid supports the dedicated Exchange hybrid app. The dedicated Exchange hybrid app becomes mandatory in October 2025 for continued cross-premises functionality (free/busy, etc.). To make the required changes related to the Graph permissions model, you have some more time, as that will become required in October 2026. For more information, please visit this link.

Do note that Microsoft scheduled some planned disruptions.This is likely in an attempt to nudge those Exchange hybrid customers who have not yet implemented the new dedicated hybrid app. So, if you are running Exchange hybrid with mailboxes on-premises and in Exchange Online, have not deployed the April 2025 SU or later, or did not implement the dedicated Exchange hybrid app, here are some time windows to keep an eye on:

• Sep16-18 (7am-7am). Affected regions: WW, GCC, GCC-H, DoD, 21Vianet
• Oct7-9 (7am-7am).

Symptoms: Users with mailboxes on-premises might not be able to see free/busy, MailTips or profile pictures from users with a mailbox in Exchange Online. Only EWS functionality is affected, thus things such as migration jobs and mail flow keep functioning.

For more information, keep an eye on the EHLO blog announcements.