Mailserver with sendmail!!
Its always a good idea to have dns properly setup before you try fooling around with sendmail. One reason is if you do not have a proper PTR record with your isp you will get blocked by most major email servers since they rely on that as a level of protection agents spam.
This is a great article which i found very helpful.
http://linuxjournal.com/article/5507
Take notice that in the beginning the user helps you setup your server so it will listen to the external network. Until you feel comfortable i would leave it so that the only host able to send mail is the localhost so u dont leave yourself open to spammers.
Google.com
sendmail.org
Freebsd.org/handbook
netbsd.org
Above are good ways to learn about sendmail. Even if your using linux the Freebsd/netbsd handbook a documentation will give good detail of files you will need to be familiar with.
Good luck!