[BritTim]

Fastmail has the ability to support incoming (and sent) mail in separate hierarchies. It takes some initial setup, but almost anything is achievable. Typically, you want separate inboxes and sent folders by default for each category of mail, with additional folders below the basic folders for holding mail, for instance, from specific important customers, or from suppliers.

A couple of times, I have set up accounts for entrepreneurs who have distinct businesses where email must be carefully segregated and (in particular) where sent mail must specify the correct reply address to maintain the separation. It must all be automatic (once correctly set up) because those people are very busy.

One client has over 700 folders separated into six hierarchies! It works! The only issue is that email to the backup account (which is not Fastmail) is not organised in the same way. Should Fastmail go down for more than a few minutes, and it be necessary to access mail from the backup account, finding old messages is not efficient although it is all there. Luckily, that is rarely necessary.