I need to make a confession about my terribly unprofessional project I made. Around two years ago I got thrown for the first time into back end development - I had to work on the project alone. As a very smart man I basically exposed our SMTP server as a nice and very flexible API.

Fortunately it was, by the design, a very short-lived project, taken down from the web completely and for good after around 2 months. I'm still happy I had more luck than brains and nobody used our server as a spam sending service in our name and I have learned a valuable and relatively cheap lesson in security this way.