JMAP is intended to be a new standard for email clients to connect to mail stores. It therefore intends to primarily replace IMAP + SMTP submission. It is also designed to be more generic such that it can be extended with contacts, calendars in the future (replacing CardDAV/CalDAV). It does not replace MTA-to-MTA SMTP transmission.
It’s too hard to write a good MUA with current standards, which has led to a stagnation in good email clients and a proliferation of proprietary protocols.
Current and planned implementations: http://jmap.io/software.html