You can use https://stalw.art. Compared to other email stacks, it is very easy to set up. Its support is on par with Fastmail.
I develop a JMAP email app (https://mailtemi.com) and have run equal tests against stalw.art locally as well as Fastmail.
But there’s no calendar or contacts support with stalwart right?
Neither Fastmail's JMAP for third-party apps nor Stalw.art supports Contacts/Calendars yet.
Fastmail said that it will be enabled once it passes IETF standardization.
Stalw.art also mentioned they will add support after the standard is finalized.
How come there are no free of cost email providers that support JMAP?