[Exim] Using Spam Assassin with Exim for remote addresses

I'm trying to set up a Solaris 8 (Intel) box to act as a spam filter wedge
between anb external and internal mail servers. Essentially, the external
box would forward messages to the wedge through MX addressing and the wedge
would forward doing the same.

What I was using as examples was the following director for forwarding:

forwarddomain:
driver = smartuser
domains = testshield.xyzzy.com
new_address = $local_part@???

..and the director from the docs from Andi Gohr for using Spam Assassin for
filtering:

#Spam Assassin
spamcheck_director:
condition = "${if eq {$received_protocol}{spam-scanned} {0}{1}}"
driver = smartuser
transport = spamcheck

...along with the corresponding transport:
#Spam Assassin
spamcheck:
driver = pipe
command = "/usr/local/bin/spamassassin ${sender_address}
${pipe_addresses}"
prefix =
suffix =
check_string =
escape_string =
# for debugging change return_output to true
return_output = false
return_path_add = false
user = root
# group = mail
path = "/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin"
current_directory = "/tmp"


This didn't seem to work in that the forwarddomain director overrode the
spamcheck one. So I tried to combine the two directors and came up with the
following:

forwarddomain:
condition = "${if eq {$received_protocol}{spam-scanned} {0}{1}}"
driver = smartuser
domains = testshield.xyzzy.com
new_address = $local_part@???
transport = spamcheck


It was giving an error with respect to delivery in that it was looking for
/var/spool/mail/<filename>. I updated the perl script to point to the
Solaris default of /var/mail/<filename>. It then was looking for a nobody
MBOX which I just touched a file to create. However, it looks like it wants
to deliver all mail to /var/mail/nobody. I'm not sure what the way around
this is. Is there any way of just using Spam Assassin as a pipe and
rewriting the delivery address either before or after the message is
filtered?