Transport rules aren't applied to multipart or alternative messages

Applies To
Exchange Server 2019

Symptoms

When you authenticate and submit a Multipurpose Internet Mail Extensions (MIME) message, the message is marked as Unknown type, and transport rules are not applied.

Resolution

To fix this issue, install the following cumulative update:

Cumulative Update 14 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019