There is a spot where you can register an account portfolio and add your Receipt numbers and you click on the spot for "email notifications". Or more specifically - "E-mail On" and "E-Mail Off". When you register - you give them your e-mail address.
https://egov.uscis.gov/cris/jsps/index.jsp
- Click register and follow the steps to create a portfolio and then add your cases. That way you can log in and see all your cases, you don't have to enter the case nubmer every time you want to see an update.