How to Specify the Hosts That You Want to Openly Relay to All Domains in Windows

This step is for hosts, which are most likely your internal servers that would need to send to all domains on the Internet. It is not recommended to not have any restrictions because anyone can use your server as an open relay. It is recommended to only allow the minimum, necessary hosts to openly relay to all domains. To do so:

1. Open the properties of the Default SMTP Virtual Server.
2. On the Access tab, click Relay.
3. Click Only the list below, click Add, and then add the hosts that need to use this SMTP host to send email. On the dialog box that appears, you have the following options:
   • Single computer: Specify one particular host that you want to relay off of this server. If you click the DNS Lookup button, you can lookup an IP address of a specific host.
   • Group of computers: Specify a base IP address for the computers that you want to relay. You have to specify the octets in the IP address for hosts that you will allow to relay. For example: If the IP address is 192.68.7.21, and you want any hosts with the first two octets 192.68 to relay, specify 255.255.0.0 for the subnet mask.
   • Domain: Select all of the computers in a domain by domain name that will openly relay. This option adds processing overhead, and might reduce the SMTP service performance because it includes reverse DNS lookups on all IP addresses that try to relay to verify their domain name.

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 Thanks to http://techgurulive.com/2009/03/16/how-to-specify-the-hosts-that-you-want-to-openly-relay-to-all-domains-in-windows/