[exim] SMTP Authentication

Hi at all,
i'm newbie to exim. I've installed exim on my pc, i have debian linux
distro. I wanto to realize SMTP authentication in sucnh way that authorize
only certain user to relaying email (hope this is correct). I have read exim
authentication to realize plaintext authentication with passwd file, but i
haven't understood how to add users to file, or rather i know that must add
users as : user:pwd , but how i crypt password for each user? Is there any
tools that do it ??

Can i use postgresql dbms to store authentication users ??
If I store users' authentication as above, could I not add users to the
system?

Please help i'm in trouble, any kind of help is granted.
regards
Mulp