Are you seeing this error in one of our apps?

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 real customers testing emails.

Here is how you can turn email on in your sandbox:

https://help.salesforce.com/articleView?id=000322724

Did this answer your question?