Need mail server help

[Michael Sullivan]

Hi.  I'm running Red Hat 9 and I have a domain (espersunited.com) and I
would like to set up my own mail server so that I could set up my own
email addresses @espersunited.com), but I don't know how to do that. 
All my searches on the internet have only told me how to set up mail
servers with existing mail addresses, but not how to set up addresses
when the email address doesn't exist yet.  Can anyone help me?  Thanx!
-Michael Sullivan-