Exchange can't decrypt IRM messages

Symptoms 

Consider the following scenario:

• In Microsoft Exchange Server 2019 or 2016, you enable a Microsoft Information Protection Client (MSIPC) stack by following the steps that are provided in Enable support for AES256-CBC-encrypted content in Exchange Server August 2023 SU​​​​​​.

• A message that is protected by an Azure Rights Management Services (RMS) template (for example, “Do No Forward”) is sent to a distribution group (DG).

• You are a member of the distribution group.

• You try to open the email message by using Outlook for iOS or Outlook for Android.

In this scenario, you can't open the IRM-protected message by using the mobile Outlook client.

Resolution 

To fix this issue, install the following security update:

• Description of the security update for Microsoft Exchange Server 2019 and 2016: November 12, 2024 (KB5044062)