[Exim] precedance of rejections

hello,

is there a way in exim 3.32 to have sender verification and
especially sender_verify_callback coming before a rejection because
the user is a unknown user on the system.

we have a lot of spam entering our queue like this : a lot of spam
with forged sender addresses is sent to non existant users of our
system (speculating on the existance of a default, catch all
instruction), these mails get bounced and exim sends immediately an
error 500 back, which the destination hosts reply with a 500 getting
stuck in our queue then.

what solutions are there to this dilemma?

--
jens-ingo