Tim:
I suspect it's merely a label and application-specific. The moment you email someone without the requisite software it's going to get ignored.
Stephen Morris:
Organisations, if they so desire, can put processes in place to handle mails with headers specifying sensitivity differently to mails that don't have the headers. In fact, I'm trying to get a defect resolved at the moment where somewhere between the code I wrote to send the mail and the mail arriving in Outlook 365, the sensitivity level in the headers has been changed.
I think you'd have to have a custom mail server to enforce things. For enforcing you have to take control out of the hands of clients.
Big oops, I missed it. In the message editor for new message or replies (on my old CentOS 7 installation)
Insert menu blah blah blah blah Custom header blah blah blah blah
And a slightly different position in the Insert menu on my Fedora 40 installation, so I suspect it's still there in newer releases.
Click on the custom header menu item, and a window pops up
Email Custom Header Security Personal Unclassified Protected Confidential Secret Top Secret None
On a whim, I picked "unclassified" for this reply, to see what it does, and if it makes it through the list server.