I haven't tried this myself but using SEND_MAIL try putting
Sensitivity: Private
As the first line of the body followed by a blank line before the rest of the body.
It's probably a long shot but might work.
Original Message:
Sent: Jun 02, 2023 06:06 AM
From: Florian Ley
Subject: Set outgoing mail as 'private' (SEND_MAIL)
Hello there!
I'm not finding a way to set an outgoing email as 'private', is there any solution? Function SEND_MAIL doesn't have a paramter for that right?