Re: Back-queries?

I guess the idea isn't to check whether the sending MTA will accept
mail for the from address, but to check whether the sending MTA is
an open relay.  The problem with this approach is that you can't
tell whether an MTA is an open relay by looking at the response to
RCPT - even a closed relay might return 2xx in response to RCPT 
and then bounce the message.

Keith