Another quick post to demonstrate how to check if your Exchange server or other mail relay has been configured for open relay.
If your internet facing SMTP server is an open relay then that means it'll relay email from any address to any address where the email originated from any IP. This will cause problems because it means that anyone on the internet can send spam email through your SMTP server and blacklist your domain or IP.
An example of the PowerShell script output is below. :
The machine you run this from needs to be Server 2012/Windows 8 or higher.
To view more information and instructions on how to run the script, click this link: https://gallery.technet.microsoft.com/PowerShell-Open-Relay-Test-d1b562dc.