sendmail (or exim) help, please

[runner@matavnet.hu]

Hi !

Is it possibble to masquarade my e-mail to the outside world from 
my LAN ? I'll tell you exactly what I wanted to say. I have a local 
LAN, one e-mail address. I have set up a local DNS and ipchains 
rules but I stopped at sendmail. I want to relay mail for the local 
machines and put the mail into a queue if it goes to the inet but 
deliver immediatelly inside the LAN. The domain has the name 
"linbase.org" (not registreted) so all of the outgoing mails have to 
have "runner@matavnet.hu" (the valid mail address) at the From: 
and in the Reply-to: fields, not someone@linbase.org.
Please, help me, how to setup this ! I welcome any URLs, RTFMs, 
HOWTOs if it's apart from the "official" docs in the distros, 
because I have already read through them but (maybe I'm too dumb 
or overlooked something) found nothing to my special problem. 
Does anyone has a working solution to the situation like this ? If 
you have a solution with Exim I would welcome it too.
Thanx a lot,
Ago