I really hope Fastmail implements the JMAP spec for calendars and contacts soon. They’ve had the mail part of the spec implemented for a while, but it still requires CardDAV/CalDAV for contacts and calendar access.
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The downsides of developing and testing this stuff as we were writing it up!
We've finished rewriting the objectid generation to give smaller sized and more sortable IDs (they're an inverse of nanosecond internaldates now, plus some extra magic on the low bits for IMAP appends to avoid clashes)... which we wanted to speed up and reduce disk usage for the offline mode.
Next up is indeed updating to the latest spec on calendars and contacts. Files might take a bit longer, I really want to do some desktop clients for the files system, we have a really nice files backend at Fastmail which is only accessible via our interface or WebDAV right now.