I'll give you a Red Hat 8.0 Sendmail hint right off the bat (for outgoing mail only):
First make a backup copy of your /etc/mail tree. Then put this in /etc/mail:
http://voidmain.is-a-geek.net/files/sendmail.mc
Replace "voidmain.is-a-geek.net" wherever you see it above with whatever you want to show up in the "@somewhere.com". Hint, somewhere.com needs to be resolvable in DNS either with an A record or MX record. And this is *only* for sending mail. If you want to also receive mail you'll need to change one line above. I do not want to receive mail on my machine so I left sendmail listening on 127.0.0.1. For outgoing mail, many mail servers will reject your mail if it isn't from a valid domain or host that is resolvable in DNS (you don't have to own this name, it is masquerading afterall, think spam).
From the above sendmail.mc file you will build a new sendmail.cf:
# cd /etc/mail
# m4 < sendmail.mc > sendmail.cf
# chkconfig sendmail on
# service sendmail start
Test:
# echo "This is a test" | mail -s "test subject"
somebody@somewhere.com
This should get you going...