One of the signals they use for classifying spam is people clicking "Report Message->Junk" in Outlook, so it's probably a constant battle with people who forget they signed up for something and call it junk.
People forget they signed up for something, didn't opt-out, got on some list from a conference they attended, etc. and just report as Spam even though there's an unsubscribe link.
Frankly, the vast bulk of the email in my Spam folder isn't egregious fake medicine and the like. It's mostly low quality mailing lists, much of which isn't much different from what ends up in my inbox.
> didn't opt-out, got on some list from a conference they attended
I would totally consider emails I received because of either of those things as spam. If I get an unsolicited email, it gets reported as spam regardless of the presence of an opt-out link. I wouldn't click any link in an email that came from someone I didn't opt into getting email from.
Frankly, the vast bulk of the email in my Spam folder isn't egregious fake medicine and the like. It's mostly low quality mailing lists, much of which isn't much different from what ends up in my inbox.