RE: Adding a specific external contact to customer email notifcations
Hello nickmang,
to allow external, unresolved email addresses, you have an option within your System Settings, to allow these.
Please go to your Advanced Settings > Settings > Administration > System Settings:
Check the Tab "Email" and you will find an option for "Allow messages with unresolved email recipients":
After setting this to "yes", should resolving your issue.
I hope this helps. Please let me know, if this resolved your behavior.
Thank you very much and have a nice day.