I chased down a spam flag by one of our users a while ago (we only send transactional emails to actual accounts) and actually spoke to the user since he complained about not receiving any of our emails.

Turns out he uses the spam button in his email client as a temporary deletion / archive for emails he wants to read later...

TLDR; Many users create their own issues by just not giving af. It reminds me of antibacterial hand wash.