A customer at a university wanted to ensure important system messages were not put into the clutter folder in user’s mailboxes. They asked how can you whitelist these system messages.
I found a way to do it with transport rules with this example syntax:
New-TransportRule -Name <name_of_the_rule> -SubjectContainsWords "Meeting" -SetHeaderName "X-MS-Exchange-Organization-BypassClutter" -SetHeaderValue "true"
The header and header value is case sensitive so watch the case.
Here is a sample:
in the GUI the clutter ‘whitelist’ message rule looks something like this:
See more information here.