There is no plugin.yml in the .jar file, nor on GitHub.
**EDIT - you named it nukkit.yml, not plugin.yml
github.com/popkechupki/ServerMail/blob/master/src/main/resources/nukkit.yml
Even after I added one, it does not work. I compiled using Java 8.
Errors on startup:
May 16 12:21 [Server] FATAL...