Much of the spam we receive is sent through open relays and insecure mail servers.
An administrator thought his server might be compromised "I think my Exchange server is sending out spam, but I don't know how to stop it. When I look at the queues, there are emails coming from people who are not in my domain. Our domain is blacklisted by many sites. I tested the server and it is not a open relay. I'm using recipient filtering and don't allow anyone not in my domain to send. "
You need to check your SMTP virtual server configuration. Open the Exchange management console and under the Server name and Protocols you should see SMTP and under it SMTP virtual server. Select properties. There is an Authentication tab and Anonymous should NOT be enabled. Using Basic and Integrated Windows authentication is fine. Also set the default domain to your internal Windows domain.
Next check the "Relay Restrictions" box in the SMTP that allows you to specify which IP addresses can relay, choose "Only the List Below" then only those IP's that are listed will be able to send through the server. Enable the option to allow all computers that successfully authenticate to relay regardless of the above list is selected or your internal servers will be unable to send outbound mail.
