Using New Outlook on Windows for personal email, calendar, and contact management
Thanks Steve,
My sub folders are for clients, therefore the suggestion doesn't work.
That said, when I accessed my 2025 folder in new Outlook this morning, it suddenly appears to be in alpha order.
All very mysterious however perhaps the solution is to revert to old Outlook, re-sort sub folders A-Z, reboot or wait overnight, and return to new Outlook which will have replicated the folder structure from old Outlook.