Have configured Sendmail 8.14 on a RHEL 5 server. The server is running an isolated LAN not connected to the internet. Therefore, I am not using any DNS and the number of clients is small enough that I've just manually updated the /etc/hosts files. Hence, there is no listing in /etc/hosts or anywhere else on the system for something like "servername.domain"; I simply use "servername" in the /etc/hosts file. Also, I am using just the simple "mail" program as the MUA. Here's the problem.....when I try sending or receiving mail as root, it is successful. However, if I send mail from root to a user on the server or from the user to root, it fails. It complains of "500 5.1.1 Unknown user". Ideally, I'd like to be able to email from the client terminals, but for now, I am satisfied with just getting emails running on the server from email@example.com to firstname.lastname@example.org. What am I missing here?