Maybe via some method involving aliases, you can add labels and outlook seems to have some advantages over gmail. This article below explains.
The “Plus” addressing seems similar to aliases and I have briefly look at fast mail.
Although I am not wanting to have additional email accounts as this would even notifying recipients of the new address (messy), just separate inboxes for particular email streams coming into my single mailbox. So my personal inbox, my brothers affairs (guardianship), finances, family matters, hobbies etc.
I am expecting I just have to do what everyone else does create multiple email accounts if I want separate inboxes.
But I find it more interesting exercise that I went left (single email account) wanting a separator of the email streams. While traditional everyone else went right (multiple email accounts) with a consolidator.