Configure a server as an SMTP Relay Server or Smart Host – Setting up an SMTP server on an IIS to redirect mail to an Exchange
This procedure explains how to mount on an additional server on your network (or the DMZ) an SMTP service and pass all of your organization's mail through this server, for the service we want, either an AntiVirus/AntiSPAM server… Logically, all SMTP traffic will pass through it, both incoming and outgoing.