Sendmail on a LAN

[JD]

  I read the wiki for setting up and configuring sendmail
http://www.wikihow.com/Configure-Sendmail

Clearly, a full setup of DNS server for your domain
must be set up, per this wiki, along with mx records ...etc.

Does this prevent one from settiing up and using sendmail
on a LAN to send and receive email to/from the outside world?

The main router is set up to forward all necessary ports
(smtp, pop, pop3, ....etc) to the machine which would run
sendmail. The firewall on the machine is set up to allow
packets to/from these ports.

I understand that some things need to  be set up so that sendmail
sends headers that use a routable IP address as the source of
the message. Is it possible to make sendmail use my router's
public IP address in the message headers? How?