My apologies in advance for this newbie question, but I need some
clarification so that I understand Sendmail's role in the mail server setup.

The docs say that Sendmail is not a "final delivery agent".

Just exactly what does that mean?

Also, I have a default configuration of Sendmail 8.9.3 setup and can telnet
to port 25, so Sendmail is at least responding.

But when I point a mail client to that box for mail (using an account setup
on the Linux box) I get a server error saying relaying is denied.

The things I guess I'm finding a mystery are:

1.  How do I setup what machines can use Sendmail (or more properly, this
box) as a mail relay?

2.  Does each mailbox require a separate user account on the Linux box?

3.  We have DNS pointed to us from an org in KC and we're hosting our web
and mail onsite. How does this affect the hostname setup that I use and what
should I more properly use?

4.  How should I properly setup this box as far as hostname/domain goes?

Our domain is atchisonkansas.net.

Thanks a bunch, all, and again, sorry if I didn't make myself clear in any
of my questions. 

Chris