Why are external invites not being sent when adding an external invitee?

Issue: When attempting to send invites to external clients, you do not receive the following prompt

Result: The external client does not receive the email with the attached .ics appointment file.

How to fix:

- Open Apple ==> System Preferences ==> Security & Privacy ==> Privacy tab

- Unlock the padlock in the bottom left corner

- Put a checkmark next to Mail under Daylite