I too am having issues where spammers are sending spam to non-existent
email addresses of domains which I host. Qmail is bouncing the email
(non deliverables) to an email address which the spammer provided as
their "mail from" address.

I need the simplest way to resolve this issue.

1.) figure out the emails they are sending to by reading the /var/
qmail/queue/mess/ and then go put a .qmail-name file in the user's
directory with a "#" in the .qmail file. I believe this will prevent
a bounce response. Please confirm if this is true. Otherwise would I
put "/dev/null" in this .qmail file?

2.) I need some sort of program which will run before qmail which will
authenticate incomming email and if the address does not exist on our
system then it should not even allow qmail to receive it and bounce
the message with the body of the message.

I would greatly appreciate the HELP from someone as our mail server is
getting overloaded with spam

Thanks,

David