Mark it as such.
And, as others have noted, unsubscribe cannot involving going and logging into their system. If I need to do that, it generally goes directly to spam.
My gut says unsolicited marketing emails, from popular sites I've never used before, like Brooks Brothers or Robinhood (especially after a "Welcome to ${site}!") or US public school event notification emails are all probably legit mistakes.
I could see even a public school system having issues with getting flagged as spam if they don't include an easy method to unsubscribe because then marking as spam+blocking becomes the best option in response to wrong address.
Which of course you click and then have to go through a number of hoops to log in, confirm Email address, authenticate, etc.
At this point, I just mark those as spam as well.