majority of spam emails are using forged addressing, propose that receiver
server connect to the sending server and see if the server bounces on that
I'm doing this now for one user community, with mixed results. In particular,
I'm surprised at the number of users whose email addresses are valid (in that
if you send them mail, it will get there eventually), but for whom attempts
to validate their addresses via SMTP fairly consistently fail for temporary
reasons (e.g. DNS timeout, SMTP doesn't respond, SMTP returns 4xx).