This is what it looks like in Outlook's Sent Items folder:
The above e-mail was sent with "Add a digital signature to this message" checked, but not the "Send this message as clear text signed" checked.
And this is what it looks like in the SM web interface Sent Items folder (raw data)
From: "Robert Simpson" <firstname.lastname@example.org>
Date: Thu, 28 May 2020 09:06:50 -0700
Content-Type: multipart/mixed; boundary=fcc03c411c224a95a85703af64480075
This is a multipart message in MIME format.
There's quite a lot wrong with this. If I check the "Send this message as clear text signed" as well, I get a different output, but it's still wrong, and Exchange still can't understand it.