Have a look here: http://www.howtoforge.com/perfect_setup_suse_9.2_p5
If you don't use a control panel like ISPConfig, you will have to set up your mailboxes manually. This is described here: http://www.howtoforge.com/forums/showthread.php?t=2
I would like to use a domain which doesnt exist but be able to send email on the LAN or even outside.
You should set up a DNS server for your local network only where you create the DNS records for your non-existant domain, and all your computers (including your mail server) in your local network should use this DNS server as their primary DNS server, because otherwise the domain can't be resolved.