manual

Getting Started Guide Installation for SMTP Servers  87
Screenshot 72 - Configure the domain
3. Select the Forward all mail to smart host option and specify the
IP address of the server managing emails in this domain. IP address
must be enclosed in square brackets e.g. [123.123.123.123] so to
exclude them from all DNS lookup attempts.
4. Click OK to finalize your configuration.
Step 4: Secure your SMTP email-relay server
If unsecured, your mail relay server can be exploited and used as an
open relay for spam. To avoid this from happening, it is recommended
that you specifically define which mail servers can route emails
through this mail relay server (i.e. allow only specific servers to use
this email relaying setup). To achieve this:
1. Go to Start ► Control Panel ► Administrative Tools.
2. Click on Internet Information Services (IIS) Manager.
3. In the left pane, expand the respective server node. Right click on
Default SMTP Virtual Server and select Properties.
4. Click on the Access tab and select Relay.