[Exim] SMTP auth

Hello,

I am setting a small mail server on our cable modem. I have it set up
and working, now I just need to configure it the way I want. Right
now I am running Debian 2.2 rev 3, with updates from the stable tree.
I am able to send mail from inside our private network, but I need
SMTP auth for roaming users. I know I could add domains or IPs, but
they are both dynamic. PLus they usually use major ISP's and I dont
want anyone from that ISP being able to relay mail. Is there ant good
tutorials or HOWTO's that explain how to do this in plain english? I
can't seem to find any, but I could just be looking in the wrong
places.

Thanks in advance,

Erik