Protecting Windows SMTP service against the spam

Question

I'm trying to use a Windows SMTP service on Windows Server 2008, but I can't understand how to secure it.

Basically, if I keep Connection and Relay settings of SMTP Virtual Server to "All except the list below" (with an empty list), a few minutes later I see spam appearing in Queue directory.

Now, if I set Connection or Relay to "Only the list below", specifying the range of local IPs, I can't use the SMTP server nevermore (a "Unable to read data from the transport connection: net_io_connectionclosed." exception is thrown if Connection is set, and "Mailbox unavailable. The server response was: 5.7.1 Unable to relay for ..." - if Relay is set).

So what is the way to get rid of spam from internet but let send mails from local network?

Answer 1

not exactly what you're looking for but you can always use built-in firewall and limit tcp/25 connections only to the local network.