Ubercart Stock Notify replaces the "Add to cart" button in Ubercart with "Notify when back in stock" when a product is not in stock and the user has permission to use notifications. An email is sent to the users when the product stock level is set above zero on the edit product stock page.
Anonymous stock notifications are supported - users who are not logged in are prompted for their email address. The module is designed to handle multiple SKUs for a product, but attribute support is currently not implemented.
This module only hides the "Add to cart" button and does not prevent out of stock items from being purchased or added to the cart in other ways, but uc_multi_stock can be used to do this.
The email subject and body can be configured at at /admin/store/settings/stock. Workflow_ng integration is included so you can add further conditions and actions for when a product comes back into stock.
The email notification message and any conditions you wish to add can be fully configured through Conditional Actions.