And I dumped them because their client was buggy, doesn’t work offline on mobile devices and I had to use IMAP anyway…
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.