i wish they did have a feedback loop. i have a catch-all domain and an employee that was terminated with us left so now I get the barrage of emails to a general box for his LI account.
well he never updated his linkedin to a new email and you have to sign in to unsubscribe. well, i don't want to reset his password because i can see he still uses the LI and there is no way to stop these emails. seems like making it impossible to unsubscribe would violate CAN-SPAM.
DKIM is not a feedback loop. A feedback loop is when someone hits spam on one of your emails, the provider lets you know what message caused the user to hit spam. If you have a unique message id or something else in the email encoded in you can tell who it was and not send them any more emails.
I imagine this is most useful for the bulk email sending services.