[Exim] Authenticators for EXIM

I'm trying to setup Mail Authentication for EXIM 3.35. And it's giving me a
heck of a time.

I first tried PAM, using the setup I had seen posted on the list before,
namely:

login:
driver=plaintext
public_name=LOGIN
server_condition="${if pam{$1:$2}{1}{0}}"
server_prompts="Username:: : Password::"
server_set_id=$1

and setting /etc/pam.d/exim as:

# This is a sample authpam configuration file that uses pam_stack
# (circa linux-pam 0.72).

auth       required     pam_nologin.so
auth       required     pam_stack.so service=system-auth
account    required     pam_stack.so service=system-auth
session    required     pam_stack.so service=system-auth

And I'm getting authentications rejected when the password is correct, or
when it's incorrect. I'm at a stand still. Anyone got any ideas? This is
RedHat 7.2 if that matters.

My PREFERED authentication method would be MySQL but I'm willing to fall
back to a password file or Pam for now.

Thanks!
-- Dan