After reading the article I can't understand, the 3 statements below seem to be conflicting :
1> We can't uninstall the last Exchange Server because this would remove needed configuration objects and management tool package would not work.
2> If we never had an Exchange server (this means configuration objects are not there), we can use the management tool package.
3> If we uninstalled the latest Exchange Server a long time ago (this means configuration objects are no more there), we can install the management tool package on a new domain member machine and use it
Concerning keeping a server onprem for MFC and apps email relaying, I know IIS 6.0 SMTP engine is out of the game, but this would be a nice option. I would not use an Exchange server for email relaying. It would be buying a Ferrari to drive in narrow streets 🙂 Open sources alternatives like HMail Server will be lighter and more flexible.