This also pre-dates organized crime becoming heavily involved, and pre-dates the obsession with browser exploits. Back then a lot of spam was sent by semi-legitimate marketers from the US. These days all the bad guys are out to get you to click on a single link.
Right. Back in the 90s spammers were trying to build their lists, and used fake opt outs to do so. These days through a combination of web scraping and dictionary attacks, they have more addresses than they know what to do with.
My advice to people these days is to unsub if a message is from someone you've corresponded with before, or if it looks like someone who is legit but clueless. Then hit the spam button.
Regards, John Levine, jo...@iecc.com, Primary Perpetrator of "The Internet for Dummies", Information Superhighwayman wanna-be, http://www.johnlevine.com, ex-Mayor "More Wiener schnitzel, please", said Tom, revealingly.