Product Notifications
From Shopnix Help
Whenever a product is out of stock, the button 
in the product page will be change into 
. Your store customer(s) can click this button to submit their email id to get notify when the product will avail in the stock.
When the product will be back in stock i.e. you change the stock level in the Shopnix store manager, then an e-mail will be send automatically to all the customers who subscribed to be notified. This e-mail will indicate that the product has just come back in stock and can be bought right away.
To access all such submitted email ids, login to store manager & go to Admin > Products > Product Notifications.
You also can add the customer’s mail id manually for notifying him for product availability. By Admin > Products > Product Notifications >Add New Notification.