I will go over the steps to create an internal SMTP relay using Windows Server 2012 R2 and IIS 6.
8 Steps total
Step 1: Install SMTP Service
Open the Server Manager and choose Manage and then Install Roles and Features. Click next till you get to Features and scroll down to SMTP Server and check the box. Hit Ok to all the dependencies and click Next then finish. Let the installer run to completion.
Step 2: Open IIS 6 Manager
Open Tools then IIS 6 Manager
You will find the Local Server listed that you just installed SMTP on. Right click on the Local Server name and choose Properties
Step 3: Set the IP address of the LAN device
In the Properties menu click Access at the top then Relay at the bottom. Set the individual IP addresses of the LAN devices that need SMTP relay without authentication.
Step 4: Delivery Tab
Click the Delivery tab at the top of Properties and choose Outbound Security
Click the radio button next to Basic Authentication and set the smarthost user that will be used to send email. If you need TLS choose that check box at the bottom.
Step 5: Delivery Tab
Choose Delivery Tab then Advanced at the bottom
Set the server URL for the outgoing server that requires authentication. Gmail, outlook.com, ISP smtp servers can all be used to send email.
Step 6: Messages Tab
On the messages tab you can set email size limits according to what you are sending. I usually raise these limits for scanners and stuff.
Step 7: Check Firewall
Check the Windows Advanced Firewall and be sure that SMTP in is allowed so you can get traffic to the SMTP service.
Step 8: Set Devices
Go around and set all the devices that need SMTP relay to point to the new SMTP server you just setup and test.
Really easy to do but not obvious to most how to do it properly. Hopefully this How-To will help you get your SMTP relay going.