Configure IIS to be a Smart Host for Exchange Server

Step 1: Verify the Installation of the SMTP Service
In Control Panel, open Add/Remove Programs, click Add/Remove Windows Components. Click the Internet Information Services (IIS) component, click Details, and then verify that the SMTP Service check box is selected.
If it is not selected, click to select it, click OK, and then follow the installation directions that are displayed.

Step 2: Configure the SMTP Service to Relay for Internal Domains
Depending on the scenario, it may be necessary to configure the SMTP service to relay inbound messages for your internal domains.
1. Click Start, point to Programs, click Administrative Tools, and then click Internet Services Manager.
2. Expand the tree under the server name, and then expand the Default SMTP Virtual Server. By default, you should have a Local (Default) domain with the fully qualified domain name of the server.
3. Configure the domain for inbound:
a. Right-click the Domains icon, click New, and then click Domain.
b. Click Remote, click Next, and then type the forwarder.yourcompany.com in the Name box. Click Finish.

Configure the domain for relay
1. In the properties for the domain that you just created, click to select the Allow the Incoming Mail to be Relayed to this Domain check box.
2. If this is being set up for a internal domain, you should specify the server that receives e-mail for the domain name by the IP address in the Route domain dialog box.
3. Click the forward all e-mail to smart host option, and then type the IP address of the server that is responsible for e-mail for that domain in square brackets.
Note: Typing the IP address of the server in brackets is necessary so that the server recognizes this is an IP address and not to attempt a DNS lookup.

Step 3: Specify the Hosts That You Want to Openly Relay to All Domains
Note:
Anyone can send to the domains that you specified in Step 2.

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, make sure the list is empty. Check the checkbox before “Allow all computers which successfully authenticate to relay, regardless of the list above”.