create email id for registered users in Drupal 7 and Nginx
Nginx won't help you. It can act as a mail proxy. But you need an MTA: Sendmail, Postfix to create a new mail account on your domain. Take a look here:

https://stackoverflow.com/questions/5273163/how-can-create-email-id-using-shell-exec-or-exec-in-php

You will require root permissions.