It looks like you're new here. If you want to get involved, click one of these buttons!
Does the most recent version of Sendy have the list unsubscribe mailto included?
Sendy's List-Unsubscribe header includes the unsubscribe link and not a "mailto" link. 'mailto' requires the subscriber to manually email you to ask you be removed from your list which is cumblesome, at the end the subscriber just might hit the 'Mark as spam' button as it is more convenient.
The unsubscribe link instantly removes the subscriber from your list when they click it.