How do you view the rest of typical SPAM filtering, where the mail is apparently accepted for delivery but then silently thrown away? For simplicity assume a system such as mine where I control the MTA and client, so no one is making decisions hidden from me as the end user who wants to get the ham and see no SPAM. (I get tens of ham per day and many many thousands of SPAM attempts.)
>>rhaksw+t
Note that in the GreetPause case the SPAMmer will not see the rejection errors since they don't look at the response to their hit and run (ie no one gets to see any error, neither sender nor target), and a legitimate sender should never get the error, so even this may be messy by your criteria I think!