[Exim] Authentication from system password

I've got the following as a PLAIN authenticator currently:


fixed_plain:
driver = plaintext
public_name = PLAIN
server_condition = ${if eq{$3}
{${extract{1}{:}{${lookup{$2}lsearch{/usr/local/etc/exim/relayers}{$value}}}}}{yes}{no}}
server_set_id = $2


This is on a FreeBSD 4.8 machine. What is the best way to change this
to refer to system passwords for authentication? I tried:

server_condition = ${if pam{$1:$2}{1}{0}}

This seemed to allow authentication without the proper password, and it
took a while to process the SMTP session. I'm guessing it failed after
something timing out and allowed authentication. Perhaps because exim
cannot access PAM if it isn't root?

What I'd like to do ideally is authenticate against system passwords,
and keep exim running as an unpriviledged user (mailnull).

Thanks!