I use apple mail and iOS mail. At one point my wife was using whatever Microsoft's email/calendar/contacts app is called but she's long-switched to apple products.
On the server I have sogo running so I also use iCal and the Mac and iOS contacts apps.
Passwords, tokens, and attachments stored on your servers according to your privacy policy. And headers, message bodies, drafts, contacts, and flags if encrypted server cache meant what it sounded like. And telemetry with no choice mentioned.
Ah, I see. Personally, I wouldn’t really consider using a vendor-specific app when the same vendor-agnostic workhorses have been doing their job fine for decades. Never used Fastmail but back when I still used Gmail, you could use it just fine with IMAP and your client of choice.