Given that SMTP is a relay system, would the first US-soil SMTP gateway machine a message travels through be its legal "sender" (in the way that e.g. the importer of a foreign medical device is liable if it's not FDA-approved)? One would think this would legally force those gateways, when handling foreign-originated spam, to slap their own address on them with their own unsubscribe link, that would—if clicked—then block those messages from coming through that gateway.