Sure, not widely supported, but if you use clients supporting it, it is great. Blazingly fast, while IMAP is always slow.
Also, Thunderbird is working on JMAP support: https://blog.thunderbird.net/2025/09/state-of-the-thunder-mozilla-connect-updates/
I only ever had had a website reject my domain once, in around 15 years of using my own domains for email. I just signed up at another website providing the same service.