You could probably implement JMAP via a Worker, but Thunderbird itself doesn't support JMAP yet. Note too that you would have to find a provider to actually send the emails, so you might as well just use them directly, rather than try to run it through a Worker...
Because Cloudflare itself doesn't provide email sending support. You can receive emails to verified email addresses with #email-routing, and you can send to said verified emails, but to be able to send emails to any email address, you need an external provider.