In my company, it's somewhat common to use "SO:" at the front of the subject, meaning "Subject only". This would address your point about MUAs truncating the su

In my company, it's somewhat common to use "SO:" at the front of the subject, meaning "Subject only". This would address your point about MUAs truncating the subject and work just as well. Maybe it would be helpful to put "SO means 'subject only'" or something in the body, which could be automated, to further decrease confusion.