They do, when a user marks an email as spam gmail pings the sender back and notifies them.

I imagine this is most useful for the bulk email sending services.

gmail does not have a feedback loop

I think you are right, I thought I had read about the feature somewhere but now I cannot find any mention of it.

Thank you for calling me out.

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.

Gmail certainly does: it implements DKIM.

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.