Sending mail to another domain

hi,

i had configured postfix mailserver in rhel4 update 4.
postfix-2.2.10-1.RHEL4.2
server hostname is experts.linux.com were i configured postfix smtp and imap.
imap-2001a-18 is which i used.

i had configured squirrelmail in another machine where the hostname is rules.linux.com
squirrelmail-1.4.6-7.el4

when i send any mail to smtp server(experts.linux.com) from squirrelmail server(rules.linux.com) its working fine.
mails are delivering to the respective users.


my problem is i had another domain (radius.linux.com).In this server only imap and postfix services are running.I had to add this server in the main.cf file of experts.linux.com.so that when i send mail to radius.linux.com users from squirrelmail it should be delivered.

please say me where to add another domain name in the main.cf file according to my setup.

my task is i had send on mail to radius.linux.com user from squirrelmail (rules.linux.com).I know that i can be done thro DNS but i need to knw without DNS how to do this?

thanks in advance.