@Silverpulser,
I have the option up in the right corner for the old Mail. But I usually start the New outlook directly with the Shortcut I pinned for it.
Edition Windows 11 Pro
Version 24H2
Installed on 2/8/2024
OS build 26052.1100
Experience Windows Feature Experience Pack 1000.26052.1100.0
New Outlook works when started from the Start menu and when I press the option button it reverts back to the old Outlook. Pressing the button again doesn't work.