Emails with double opt-in still can BE spam. The act of signing up for an email list does not give the email list owner the right to send any amount of any email content to you.
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.
---
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.