[Exim] [newbie question] How to check senders ?

Hail to everyone !

I am a newbie user, and I wonder how to control mails sent through my
system. I heard about a way to check senders by using the POP daemon.

I understood that the way to do it it to make the users identify theirself
with the POP daemon, and then lets them send mail through SMTP (exim) by
verifying their IP address (and login ?) in thle logs ...

Is that correct ?

Where can I find software and documentation for that ?

Another question ...

I currently try to set up gnu-pop3d as POP daemon ... Is there any better
software, more flexible ?

Another and last question ..

If you are a Linux Redhat 5.2 user, could you tell me how do you start your
exim "daemon" : daemon mode or inetd ? If inetd, what is your config. line ?

Thanks in advance for not flaming me with my fools questions !

Georges (France).