Earlier quoted context omitted.
Definitely. I’d hate to have it download all my mail on my laptop or my phone. We have yet to particularly consider the UI for controlling it, but I fully expect that anything in the way of synchronisation rather than simple caching of what we’re already fetching (and only limited amounts of that—there’s already an LRU GC on the cache) will require manual configuration, probably with options like “sync the inbox” or…
I can't imagine not downloading all mail, why wouldn't you want a local backup? Space is cheap.
(a) it should support fully offline and session-agnostic functionality. We’re very unlikely to ever go this way on fastmail.com, because it’s just not going to be useful to almost anyone, and is more complex and easily leads to confusion. Instead, the data will be tied to your session, so that if you log out, or your session is permitted to expire, or you close your account, then the data will all promptly be deleted when you next go online.
(b) it has a way to export it again. Theoretically with JMAP we could actually do that, but even if we did implement it (and I doubt we ever will), it lacks the portability of other formats like mbox or PST.
Taking both of these points together, I say that an email service provider’s app’s sync should not be considered suitable for a backup. For convenient offline access, perhaps, but not for a backup.