Just specify which mail servers can route emails through this SMTP email relay server. (for example, allow only specific servers to use this email relaying).
SMTP Email Relay Server
1- Click the start button and click administrative tools and then select Internet Information Services (IIS), Manager.
2- Under Internet Information Services Manager and expand the GFI Mailessentials (local computer) right click on the SMTP virtual server and then select properties.
3- Select the Access tab and then click the Relay button under relay restrictions.
4- Select only the list below and then click the Add button.
5- Specify the IP Address of the internal mail servers that are allowed to route emails through your mail relay server. Add single computers or groups of computers or Domain and then click ok.
NOTE:
The Domain option adds a processing overhead which will degrade SMTP service performance. this is often thanks to the reverse DNS lookup processes triggered on all IP addresses (within that domain) that attempt to route emails through this relay server.
6- Click ok.
7- Select Delivery and then click the Advanced button.
8- Click check DNS, the domain name is valid and click three-time ok.
For more details click here
