Get User by Email in Hubot/REST

Hey there,
i try to write a Hubot script that passes information (provided by a webhook) from another system to a specific user via DM.

The usernames are completly different, but all e-mail-adresses are the same.
So my question is:
How do i get the username/userid with an e-mail address provided or is there a way to send a DM to a user by e-mail address?

I am using mongo:3.2, and hubot-rocketchat:latest in the same Docker for Windows

Friendly greets and thanks in advance! :slight_smile: