If you are testing one of our apps in a sandbox and aren't receiving email, you may just need to change your email deliverability.
You might just not get the emails from the app. Or, you might see this error when trying to send an email: NO_MASS_MAIL_PERMISSION
This is EXPECTED behavior in Salesforce sandboxes because Salesforce blocks all emails by default in new Sandboxes to protect you from accidentally sending test emails to real customers.
To turn on email in a sandbox, go to the Deliverability page in Setup (use the quick find in the setup menu) and make sure access is set to All Email.
Didn't work? See more details and other potential issues from Salesforce help.