Traditionally I consider SPAM only unsolicited emails, any double opt-in that is too high of volume that the user could be able to opt-out. Users marking as SPAM and decreasing reputation because of that seems like the wrong target.
Sometimes I opt-out of email lists, but then I keep getting email from then, probably because I was on multiple email lists and only opted out of one -- altohugh when I subscribed, I subscribed only once.
Sometimes I try to opt-out, but they ask me to login, and I don't remember my password because they asked me to put weird symbols and uppercase letters on it, while my normal login-everywhere password does not have these.
Sometimes I try to opt-out, but the link is broken.
Sometimes (this is what happens most of times) I am subscribed automatically to email lists whenever I sign up to some website. Shouldn't this be considered spam? I did not receive a confirmation email -- or maybe I did, but the confirmation email was to confirm my account on the site, not my subscription on that email list.
Sometimes the sender forgets to his opt-out link.
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The question that these cases pose is: what is the difference between "spam" and "email that can be useful to others but that you don't want to receive"?
And the answer is: SPAM, as explained in the original submission, is a global uncustomizable tag, if something is spam, it is spam to everybody, not just to you. That is not the ideal situation. We could do better, but I don't think it will be better within the email protocol, since it would be impossible to Google to calculate the spam-probability of each message according to its receiving user. The only way it to move to other protocols.
