Qmail on Linux Open Relay

I need information to close an open relay on a dedicated server I have 10 domains configured using PLESK as virtual domains I have appeared on some blacklists today and my mails are being rejected It appears that anyuser AT [login to view URL] (this is the hostname of the MACHINE not any of the domains I use for web etc) can send mails through the server When mails are sent from any of the virtual domains they have to be authenticated and this is working fine. How can I ensure mails are not sent like this from anyuser AT [login to view URL] I require the information to do the work as I have full access to the machine myself.