[Exim] HELP: Rejecting mail from unresolvable domains

Hi folks

I have this in my exim config file -

sender_verify
sender_verify_reject = true
sender_verify_hosts = !localhost:!rocklines.example.com:!uranus.rocklines.oyeindia.com:0.0.0.0/0

Hassle is, this waits for the DATA command and accepts the mail before giving a
4xx series error when accepting mail from an unresolvable domain. This is a
total waste of bandwidth when dealing with a spammer who dictionary attacks
your server :(

220 example.com ESMTP Exim 3.16 #2 Wed, 22 Nov 2000 18:21:16 +0530
ehlo hotamil.com
250-example.com Hello hotamil.com [202.169.133.50]
250 snip responses
mail from: spammer@???
250 <spammer@???> is syntactically correct
rcpt to: suresh@???
250 <suresh@???> verified
data
354 Enter message, ending with "." on a line by itself
test
.
451 rejected: temporarily unable to verify sender address <spammer@???>
(try later)

On the other hand, sendmail gives this error at the mail from: stage itself.

220 example.com ESMTP Sendmail 8.11.1/8.11.1; Wed, 22 Nov 2000 07:51:42
-0500 (EST)
ehlo hotamil.com
250-example.com Hello [202.169.133.50], pleased to meet you
250 snip responses
mail from: spammer@???
451 4.1.8 spammer@???... Domain of sender address spammer@???
does not resolve

Is there any change in the config file I can implement to reject / defer mail
from unresolvable domains at the mail from: stage?

Thanks
-suresh

--
Suresh Ramasubramanian + mallet<@>efn.org
You spamma my mailbox, I nukea da ass