Stalwart is a mail server program, not a live email server you can pay someone money to use.
One can of course use Stalwart to run a hosted email service.
One can of course use Stalwart to run a hosted email service.
> Let us handle the complexities of your email infrastructure with our comprehensive managed email server service.
But for the original question:
> Is there any email hosting out there with support for JMAP?
Stalwart really isn't a practical answer, at least yet.