The Sandbox instance of your brand is used for testing changes and updates to the software before taking the changes live in the Production environment.
If none or some users are receiving the 'Password Reset' email, email domains have probably not been properly white-listed.
There is a setting within the Cockpit section of the platform that allows for email domains to be white-listed. If you need to have this updated, please submit a ticket to support.punchh.com.