Using Classic Outlook on Windows for personal email, calendar, and contact management
THIS!!!!
THIS is the solution! Finally, it seems MS actually added the functionality, albeit without telling anyone. I can confirm that this works for me, and is persistent across multiple emails and after closing and reopening New Outlook.
That said, even this seems to be a half-measure. I have four accounts and I have to click the setting for each one, but two of the accounts are missing this button entirely. I don't know what's different about them, but it seems that even when finally rolling out the setting we've been asking for, they still couldn't get it completely right.