[exim] Exim 3 (yeah i know) E-Mail reject question

Dear folks,

I've not needed to do anything with Exim recently so
please bare with me and my (maybe not only stupid)
but also outdated Exim3 question ;-).

There is a simple mail relay server running exim3.
Nothing special. Allowed hosts may relay, no local
deliveries (except the machine itself for postmaster@machinename
and stuff like that).

What I have to achieve is a check of certain elements
upon receipt of an e-mail after the DATA part. To be
more precisely, I need to block e-mails with certain
header-fields and maybe body-parts. I guess I need
to setup some kind of condition but since this is
a relay box, I guess I would need to do this in a Router
which does not offer things new_address = :fail: ,
does it?

If anyone is willing to help me, poor IP-guy I am,
I would be very happy.

Thank you very much in advance.

Cheers
Sascha

P.S.: No, no change to get it upgraded to 4.x, unfortunately.
Don't ask why ;-)