It is an interesting idea. I am looking forward to how it plays out. I think the first challenge will be getting adoption from MTA's, smtp libraries and major

It is an interesting idea. I am looking forward to how it plays out. I think the first challenge will be getting adoption from MTA's, smtp libraries and major email providers. This can't be done in a silo (server only) because smtp takes two to tango. Both the sender and the receiver have to implement this concept to utilize it. There will for sure be a handful of spammers that can't get their malware updated right away so you would at least have a window of less spam.