You share the server with other domains. Each users, Telnet, FTP or Email is setup as a user on the server. So recieving the message
The userid you requested is already in use by someone on this server, please select another
Just means that the USERID is already taken by another domain. A user name can only be used ONCE per server. If this is a name such as sales you can use a redirect that will make the name valid and just redirect email to a valid POP box.