I received a spam message that managed to trigger the display of
a header in the preview pane I have not seen before: "Security:
🔒Encrypted"
I am not certain if the keys and/or values of the headers shown
in the preview pane are predefined by MailMate or if they can be
manipulated by the actual message. Neither "Security" nor
"Encrypted" show up in the headers displayed when viewing the
message in raw mode.
I do not publish public keys or use any encryption with
MailMate, so a sender's ability to make a message appear encrypted,
signed, authenticated, or otherwise authoritative in this way seems
like it would be a bug or a security issue.