I've programmed this interactive tutorial for a presentation I had to hold in my highschool computer science course on how the SMTP protocol works.
Instead of just reading through a list of commands and explaining them, I thought it would be more interesting to test them out directly by having a basic SMTP server and a console where you can enter SMTP commands and send mails to your classmates.
It did work pretty well with the presentation as a further explanation for some commands and the web app is still online to play around with at
smtp-tutorial.syscy.de.
While you can always use the HELP command to guide you through the process of sending a mail, it may be confusing without the accompanying presentation.
The frontend is a basic React app with Material UI components which communicates via websockets/SocketIO with the Java backend.