> and some people treat mark as spam as their delete button, certainly more than 0.1% of people. Don't ask me why.

The delete button is for saving important stuff, not spam.

https://old.reddit.com/r/sysadmin/comments/jndew2/users_who_...

https://old.reddit.com/r/talesfromtechsupport/comments/1sac4...

https://old.reddit.com/r/talesfromtechsupport/comments/1eun2...

I recall seeing this was some kind of learned behavior from lotus notes, deleted items wouldn't count in your quota, so users would keep stuff in there.