mail to another machine on the LAN

[Hiisi]

Hi, all!
I would like to send messages to another machine on my home LAN. My
current configuration is:
Main computer (F12, serves as router), is connected to the Internet via
eth0. It shares its connection for two LAN machines - one is Win XP
laptop connected to the main computer on eth1 and the other is sitting
on eth2. It is an old Pentium III charged with Scientific Linux (I want
to send mail to this machine from main computer). Currently I'm unable
to send mail to it using this command on the router:
mail <user>@192.168.3.30
where <user> is the desired user on that host and 192.168.3.30 is host
IP. The message sends but nothing appears in <user>@192.168.3.30
mailbox :-(
How can I fix it?
By the way, I'm able to send mail to that user from any other account on
that computer.
Please advise!
TIA
-- 
Disk crisis, please clean up!