I advise a company who performs opt-in email services. In order to comply with anti-spam guidelines, this company provides an unsubscribe link in its text email, and an unsubscribe button in HTML messages.
I just started using KMail, and I concluded that KMail suppresses the unsubscribe button in the HTML documents. (I see the button in the source code.) I couldn't see anything in the configuration that affects this behavior.
I joined this forum only to post this message. As someone who earns part of his living from the commercial email business, I may be an unwelcome Devil's advocate. Nevertheless, if the developers are suppressing something that is legally required in many parts of the United States, I am wondering why.