Receiving some spam messages and solving the unsubscribe problem, I revealed that the List-Unsubscribe header tag contains an unsubscribe link which actually works – in comparison to the support of the spam server. It would be great to have next to Spam button a Unsubscribe from list button button offering the link to the tag content.
Workaround: Open original message (source) and copy the URL in the tag into the browser.