[Mailtemi]

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.

[amaccuish]

But there’s no calendar or contacts support with stalwart right?

[Mailtemi]

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.