[Exim] Relay from dynamic IPs

Hi list!

1,
I have red some docs about SMTP auth, those are confused me.
I have try relay mails from remote clients, have dynamic IPs.
Can exim "connect" to /etc/passwd n' ~/shadow directly , or
as i have red as i have to make an ~/exim/passwd file (how can i make
it) ?

2,
eg.: the domain is 123.123.0.0/16
      can i set this, only thist domain 'll be SMTP authenticated.
     Because local domain and static IPs are welcome :)
3,
What have i modificate in exim.conf?

best regards
Gergely KODAJ