Don't think MacOS has JMAP support at all, so most likely IMAP. Happy Fastmail user here. Used it on a MacBook up until about 2 years ago with no issues with native Mall.app. Used it on my iPhone until I switched to Android about 3 months ago—also no issues with the iOS configuration script which sets everything up on iOS with pretty much one click—very useful also for setting up family members' phones. Found Fastmail much, much better than Gmail IMHO who has the worst IMAP implementation I have ever seen. Bugs include command like deleting/moving/etc mail not actually having any effect over IMAP. Gmail only works in macOS/Outlook/etc apps I suspect because of all the workarounds Apple, Microsoft et al have implemented to paper over Gmail's IMAP bugs. Fastmail by contrast has worked as expected on all platforms, although I now just use the Fastmail app on Android and the Fastmail web interface on Firefox/{Mac,Windows} as Android/Windows's native apps are horrible compared to their Mac counterparts.