Hi Brian :)
Assuming the server hostname is "server". 1. On Main PC use your editor to create /etc/exim4/hubbed_hosts. In it put server: 192.168.23.200 in it and restart exim4. 2. Send mail to user@server. 3. You can replace 'server: 192.168.23.200' with 'server: server.local' if avahi-daemon is running on both machines.
It worked perfectly...I just found out that "mail" does not support Maildir format, but mutt does fine I can now send and receive mail from and to my server. Thank you very much for your help! (...And for the easy step by step guide )
Thanks again Greets mo