Approval Emails are not Triggering for External users

We are experiencing an issue in the production environment where approval email notifications are not being sent to users.

Our analysis shows that approval emails are failing for all users in production. In the test environment, approval emails are working for internal users, but external/client users are not receiving them.

We tested with multiple client email addresses and the issue remains. Users are receiving regular system notifications, which indicates that email delivery is generally working. The issue appears to be specific to approval workflow notifications.

We have also reviewed the workflow configurations in both environments and found no differences.