Meh, I don't think this is that big a deal. It's a single line in the left-hand side of the folders list.

I don't have much of a problem with Outlook, even though I haven't used it in about 12 years. Besides, it has calendaring which Thunderbird doesn't. And much like products from Apple and Google, things work really well if you live within the ecosystem. Office 365 is a much nicer product than I originally thought.