HTTP RESTful push notification, email, failed

Hello everybody
I insist on RESTful API in order to connect my java powered appliance with Blynk server
I have succeeded so far to update values of various widgets, to change their properties (color, label, etc) and the rest.
Unfortunately i’m not yet able to send notifications as well as emails (both based on POST method i guess).
I have tried either with HTTPUrlConnection (quite meshy) as well as DavidWebb, a Lightweight, user friendly Java HTTP-Client for calling JSON REST-Services
Both implementations finishing execution with a “500 Internal Server Error”
I was wondering if there is any work around.

Best Regards’