This is pretty bizarre and I was running around in circles until I discovered what was happening.
When running Windows Server 2016 and hosting web sites in IIS (Web Site #1 on port 80, Smartermail on ports 81 and 9998), works perfectly until I install the Windows Server Essentials Experience Role. This is when that new role adds ports to the Smertermail Site in IIS.
As soon as the WSEE role is added, ports *80, *443, 808:*, and a few others are added to the Smertermail Site in IIS - as seen in the attached .jpg.
This of course crashes the Smertermail site and will not allow WSEE to work correctly.
Has anyone encountered this, and why would WSEE choose to add bindings to the Smartermail site in IIS, instead of creating it's own site?
Any help would be great.
Here is a screenshot of the altered bindings after installing WSEE role on the server.