Qmail is one of the most popular mail servers available for linux. I use it personally on one of my centOS servers and it works quite well. When you install it you'll surely need to add new domain. So how to do that ?
If you installed QmailToaster you can do that from graphical interface. Anyway I prefer to do that from command line. The command for this is very simple:
/usr/home/vpopmail/bin/vadddomain domainname postmasterpassword
Lets say you want to create domain named: "example.com" with postmaster password: "password". The command will look like this:
/usr/home/vpopmail/bin/vadddomain example.com password
The last step you need to do is to restart your Qmail with this command:
service qmail restart
I tested it only on CentOS but I hope it works similar on other linux distros too. It depends on where your vadddomain is located.