Well, that is where the line of solicited and unsolicited mails fall, spammers are generally unsolicited (so no previous mail address confirmation), their unsubscribe links won't work and >90% of them are plaintext with the remainder having bad or illformed HTML. 99.8% of marketing mails I get have an unsubscribe link that works because it forgot to uncheck the newsletter when signing up, those are not an issue. And those are usually HTML mails with well formed formatting.