Currently, there is no built in functionality to email to all customers or get a list of all the customers email addresses so you can email them. If you have access to your database I can give you a query to get all the email addresses and email them then.
You would want to go to the database you have for PowerStore and run this query:
SELECT UserFirstName, UserLastName, UserEmail FROM `ps3_users` WHERE 1
Note: The user may not have filled in a first name and last name in their profile so some of the results for that will be null.