I considered FM a while back but... you pay per account! $50/account no less! I separate my email into three accounts across two domains and my wife does the same and I have kids who'll have their own email addresses at some point.
I can't even begin to consider using it at the price point I presume I'd be in.
I use a single account for 5 domains, with multiple aliases per domain.
For IMAP folders you can set a default identity and email in a folder stays in that folder without cluttering Archive, like in Gmail. So in effect you can do much with a single account.
For people reading along: $50/yr gets you 25 GB; $30/yr gets you 2 GB. I'd guess 2 GB would work just fine for most people.
OTOH, my relatively new gmail account has >4 GB in it. Other accounts have 0.8 and >3 GB; none of these were used for file storage (FUSE) or anything like that.
I wonder, maybe I then just don't receive that much email? I did a quick count in the folders view, there's currently about 35k emails which take up about 1.7G of space.
I can't even begin to consider using it at the price point I presume I'd be in.