If you're setting up a service where people can register their own usernames to be used as a hostname (
username.example.com), email address (
email@example.com), or URL path (
example.com/username) within your domain, there are some common names you should avoid letting the general public register.
This is a list of all the names I know that should be restricted from registration in automated systems. If you know of others, please let me know and I'll update this page.