[Exim] Exim PAM out-of-the-box

Dear all, I read the instructions (Andreas Gietl) to integrate PAM
with exim using a custom module, but then had a try with what MacOSX
10.3 gives and found that with an exim pam configuration:

/etc/pam.d/exim:
auth    required        pam_securityserver.so
account required        pam_unix.so

I can authenticate.
I am using the standard:

plain:
driver = plaintext
public_name = PLAIN
server_prompts = :
server_condition=${if pam{$2:${sg{$3}{:}{::}}}{yes}{no}}
server_set_id = $2



Am I missing something important? Why do linux users need a custom module?

Giuliano