The name means a small or friendly or informal mail in German. It's horrible Denglisch: German infiltrated by English.
The program is an experimental mail user agent I wrote for my own benefit. I wanted a mail user agent that satisified me, and I wanted to try some programming experiments.
I am satisfied with the result: I learnt from my experiments, it was stress-free because I never had a release deadline any user other than myself, and I like using the program. Mailchen sucks, of course. Every MUA sucks. Mailchen's problems simply aren't ones that irritate me. Much.