Tool for validating sender address as spam-fighting technique?

[Len Conrad]

>Perhaps we are talking about different things, I am talking about
>systems which send me an email back requiring me to do steps a, b or c
>in order to complete delivery of the email.

that's challenge/response, which has been widely discredited for years.

SAV is a receiving MX probing the MX of sender at domain.tld for 
verification of sender as known recipient.

Len