Using classic Outlook for Windows in business environments
And two years on there is still no real solution - accounts appear in the navigation pane in the order they are entered and there is no way to reorder them for the convenience of the user.
I came across this thread when searching for a solution to this relatively mild irritant. I have resisted migrating to the new Outlook for a number of years simply because I have been using offline folders (pst files) to manage historic emails that I wish to retain. But last week new Outlook was foisted upon me by a Microsoft 365 update that deleted classic Outlook from my PC - so there's no going back (despite there being a "Go to classic Outlook" button!
New Outlook is very pretty but functionally very poor. A retrograde step from Microsoft who clearly don't care for their customers at all ...