"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."
The 3 main Open Source servers that support it:
Please see: http://jmap.io/