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If I didn't intentionally request non-transactional mail, it is spam. By definition.

Mark it as such.

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I assume the unsubscribe link is malicious - if I didn't ask to be subscribed, why would I trust an unsubscribe link? Spam baby spam.
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I unsubscribe twice (allowing for one possible bug), then spam.

And, as others have noted, unsubscribe cannot involving going and logging into their system. If I need to do that, it generally goes directly to spam.

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I unsubscribe, and immediately set up a filter to mark any email from their (sub)domain as spam. Too many sites keep spamming for a week or two after unsubscribing, that behavior deserves a reputation drop.
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For whatever it's worth, I doubt that filtering to the spam label automatically has the same feedback effect in Gmail as a manual spam mark.
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