Qmail incoming SMTP address validation.

I've got a few mail servers setup to host a number of domains.

I have a single incoming server which does scanning with spamassassin,
clam, and qmail-scanner. Once a message comes in, its scanned, and then
passed off to one of two mail servers, depending on what domain it was
sent to. Each of those servers validates e-mail addresses as soon as the
SMTP connection is opened. They do not make it to the queue. I didnt
set these mail servers up, so i do not know how exactly theyre doing
that, I'm told that they check the assign file.

All works well under normal circumstances. However the scanning server
does not validate addresses, because it doesn't have anything to compare
them to. This is OK except that means a lot of undeliverable mail ends
up in the message queue, just to get delivered, and then bounced. Which
adds more mail to the queue(server a delivers to server b, server b says
that addr doesnt exist and bounces the message, server a tries to
deliver bounce to sender, if sender doesnt exist sender's server bounces
message.... you get the idea).

What i'd like to accomplish is this:
message comes into scanning server
scanning server checks with whichever server that domain is hosted on
if addr is invalid, scanning server rejects message without scanning it
for viruses, without scoring it for spam, without putting it into the
queue.

Is this possible? If so, how do i accomplish it?

Thanks!
--
In a world with no fences, why do I need Gates?