I mostly use gmail since I think it's quite a bit better than most other options, so I'm the most familiar with gmail. And I know that when you click "spam" in gmail it will ask you if you actually mean to unsubscribe.
For my business, I've never received any complaints that someone unsubscribed and still received another email. But most people probably wouldn't bother emailing me to complain if it did happen.
I also mark "legitimate" senders who don't have a functional unsubscribe as spam.