Hello David Greenhaw,
Greetings!
Thank you for posting on Microsoft Community!
For your concerned issue regarding "All Mailboxes" option in New Outlook, unfortunately it is unavailable in New Outlook. It is a by-design behavior. We'd suggest you to toggle off the "Try the new Outlook" button in New Outlook and swith to the classic Outlook as the New Outlook is a beta version and it is basically a web-based application. The look and feel come close to the Outlook Online version some features are still missing.


I will also share feedback portal URL so that you can submit your feedback directly to our product developer team via using this Microsoft Feedback portal.
Appreciate your patience and understanding and thank you for your time and cooperation.
Wish you great success, good luck and much happiness in the future!