I know how to unsubscribe from mailing lists. I've unsubscribed from a few. Sometimes there are mailing lists that I can't seem to unsubscribe from (well, there was only one, but I can't recall which one), and I simply turn off that email address and let their pissed off list admin unsubscribe for me. Why would I do that? Because software should pass the grandma test, if possible (we'll ignore the fact that both of my grandmothers are dead). In the case of a mailing list, many lists that I am on have a footer at the bottom listing unsubscribe information. This is good. What about those lists that don't?
The "how the hell do I get off this list" complaint is far and away the most common complaint that I see. Yes, unsubscribe info is usually sent in that first email they received long, long ago. They've already deleted that email. Or forgotten about it. Or have lost it. Same thing.
But wait! Unsubscribe information is usually included in the headers.
So what? My (admittedly dead) grandma doesn't know what a header is. If I explained it, she wouldn't know how to look for them. If I showed her, she's not going to remember how to do that months later. There's an RFC about having mail user agents recognize this header info, but note that the date is 1998 and my Yahoo! mail client still doesn't know about this header. Neither does Gmail, from what I'm told.
So why not put the unsubscribe info in one line in a footer? I understand that some people don't want the body of the email altered, but I'm so used to seeing footer after footer after huge sections of reply text that adding one simple little line at the bottom to forestall the most common mailing list complaint doesn't seem like that big of a deal to me.
Yeah, there will still be a handful of people who don't notice that in the footer, but at least then you can say "even my dead grandma would have seen that."