If you want a local backup of all your mail, using a web app like ours, even if it supports fetching it all, is not a good idea, unless it satisfies both of these criteria:
(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.
(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.