Yes, it's our Parse API. You can find a complete guide on how to use it here: http://dev.mailjet.com/guides/parse-api-guide/
Any question, don't hesitate!
I'm working at Mailjet.
You can find our API documentation and our existing wrappers on https://dev.mailjet.com.
For you need, you might be interested by our NodeJS wrapper: https://github.com/mailjet/Mailjet-sendemail
Also, we would more than happy to help you write one for NodeBB.
Feel free to contact us at email@example.com.