If you like many others have Exchange 2007/2010 server and you discover that you are an Open Relay, there is a very simple command that you can run from the ems (PS) to close this down.
Get-ReceiveConnector “AnyReceiveConnectorName” | Remove-ADPermission -User “NT AUTHORITY\ANONYMOUS LOGON” -ExtendedRights “ms-Exch-SMTP-Accept-Any-Recipient”
Replace “AnyReceiveConnector” with the name of your Receive Connector and then run the command.
After doing this you can visit MXToolbox or ExchangeConTesting for testing.