Introduction

In April 2025, Microsoft released critical Hotfix Updates (HUs) for Exchange Server 2016 and 2019, introducing significant changes and addressing key issues. These updates are pivotal for IT professionals managing on-premises Exchange environments, especially with the impending end of support for these versions.

Key Features of the April 2025 Hotfix Updates

Dedicated Exchange Hybrid Application in Entra ID

A major highlight of the April 2025 HUs is the introduction of a dedicated Exchange Hybrid Application in Microsoft Entra ID. This initiative is part of Microsoft's Secure Future Initiative (SFI), aiming to enhance the security of Exchange Hybrid configurations by separating the identities of Exchange Server and Exchange Online. The transition involves moving from legacy Exchange Web Services (EWS) API to Graph API calls with granular permissions. Organizations utilizing Exchange Hybrid configurations are required to implement this change between April 2025 and October 2025 to maintain functionality such as Free/Busy sharing, MailTips, and profile picture sharing. (techcommunity.microsoft.com)

Resolution of Time Zone Exception Issue

The updates address the "InvalidTimeZoneException" error that emerged after the November 2024 Security Update. This fix ensures the stability and reliability of Exchange Server operations, particularly in environments affected by time zone discrepancies. (support.microsoft.com)

Known Issues and Workarounds

Post-update, some users have reported that the Edge Transport service (EdgeTransport.exe) stops responding and subsequently restarts when Exchange Server attempts to decrypt messages protected by Azure Rights Management (Azure RMS). Microsoft has acknowledged this issue and provided a workaround to mitigate its impact. (techcommunity.microsoft.com)

Implications of End of Support

Both Exchange Server 2016 and 2019 are scheduled to reach their end of support on October 14, 2025. Post this date, Microsoft will cease to provide:

  • Technical support for issues.
  • Bug fixes affecting server stability and usability.
  • Security patches for vulnerabilities.
  • Time zone updates.

Continuing to operate these versions beyond the support deadline poses significant security risks. Microsoft advises organizations to either migrate to Exchange Online or prepare for an upgrade to Exchange Server Subscription Edition (SE), expected to be available in July 2025. Upgrading to Exchange Server 2019 CU15 is recommended to facilitate a smoother transition to Exchange SE. (techcommunity.microsoft.com)

Recommendations for IT Professionals

  1. Install the April 2025 Hotfix Updates: Ensure your Exchange Servers are updated to the latest HUs to benefit from new features and fixes.
  2. Transition to the Dedicated Exchange Hybrid Application: Implement the dedicated Exchange Hybrid Application in Entra ID before October 2025 to maintain hybrid functionality.
  3. Plan for Migration or Upgrade: Develop a strategy to migrate to Exchange Online or upgrade to Exchange Server SE ahead of the October 2025 end-of-support deadline.
  4. Monitor for Known Issues: Stay informed about any post-update issues, such as the Edge Transport service problem, and apply recommended workarounds as necessary.

Conclusion

The April 2025 Hotfix Updates for Exchange Server 2016 and 2019 are crucial for maintaining the security and functionality of on-premises Exchange environments. With the end of support approaching, it is imperative for organizations to act promptly by applying these updates, transitioning to the new hybrid application model, and planning for future migrations or upgrades to ensure uninterrupted and secure email services.