[Exim] checking local sender is valid local user

I have written a system filter to check if the user from a local system
is a local user and fail if not.

# EXIM filter
if first_delivery then
if $sender_address_domain contains "abdn.ac.uk"
   and "${lookup{$sender_address_domain} lsearch{/etc/exim/local-domains}\
        {yes}{no}}" is "no"
   and "${lookup{$sender_address_local_part} dbm{/etc/exim/aliases} \
        {yes}{no}}" is "no"
   then fail
endif

I would like it to reply immediately to the client via SMTP
with an SMTP error response but all it seems to do is accept
the message and then freeze it if the sender was bad.

This is to catch badly configured mail clients on PCs

Is there any way to do this?

John Linn