This is silly. There is one simple way to get rid of all joe-jobs for ever: You have to bounce the invalid recepients during the SMTP handshake, just like you do with invalid relays.
It is simple and effective, but it will take a long time before mail servers all implement it. Changing the Internet takes time, just like changing the open relaying took many years even with the aid of blacklisting. But there is no need to break how email works (SPF) or computationally intense crypto (most of the others).
If sendmail, qmail and postfix could lead and start it this way in their default configuration I'd say we would have a pretty good start.
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