[Exim] POP before SMTP

Hello,

I need to remove my exim email server from the open relay database.
The thing is that people using it are from several places on the Internet and that's why I can't restrict the access to the LAN.
I read that the solution was to use POP before SMTP.
I downloaded exim-pop.tar.Z file and I'm wondering about its efficiency.
Let's say someone connects in the morning to send an email BEFORE checking them, which means accessing SMTP before the POP - then if I understood well the principles of this system, this person's IP address won't be logged into the IP addresses accepted for SMTP connection.
And that's what happen when u use Outlook Express, which sends first the email before checking them.

Any advice?
Also I wanted to know if someone managed to use these scripts successfully under Linux Slackware 7.

cheers
Guillaume