Note I've designed and built a complex messaging system that runs over HTTP over unreliable connections and run it in production for 15 years so I know what can and does go wrong.
Popcorn at the ready.
To some degree, that isn't true anymore as of last week but of course, you meant when you used it, not generally: https://www.fastmail.com/blog/offline-in-beta/
Useless to me. Seriously.
I write a lot of email on planes and do a lot of email management.
All that works on the default Apple stuff as shipped other than account management. Using IMAP and iCal etc.