FastMail. It's one of the few third party hosts to support push email on iOS with the native Mail app (it's a custom protocol based on APNS), since Mail doesn't implement IMAP IDLE [1].
They are also the main sponsors behind the JMAP protocol [2] and some open source projects such as the Cyrus IMAP server.
They are also the main sponsors behind the JMAP protocol [2] and some open source projects such as the Cyrus IMAP server.
[1]: https://fastmail.blog/2016/12/21/what-we-talk-about-when-we-... [2]: https://jmap.io