JMAP (JSON Meta Application Protocol) is a protocol for synchronizing data between a client and a server. It is designed to be simple, efficient, and easy to use, providing a common, language-agnostic way for client applications to access and manipulate data stored on a server.
JMAP is designed to be an alternative to other protocols such as IMAP (Internet Message Access Protocol) and SMTP (Simple Mail Transfer Protocol) for email, and CalDAV (Calendaring Extensions to WebDAV) and CardDAV (Address Data Access and Versioning) for calendar and contact data. It aims to substitute email-related standards that have existed for over 20+ years.
JMAP is potentially significant because it provides a more modern and efficient way to access and manipulate data stored on a server. It is also intended to be easier to use and more flexible than existing protocols, which might make it more appealing to developers and users.
Additionally, because JMAP is language-agnostic, it can be used with any programming language, making it easier for developers to build applications that interact with data stored on a server.
Ok, cool; so when? It's up to the Gmail, Microsoft, Yahoo, and 🍏 s of the world. They control the email space and have invested heavily in their own APIs, such as the Gmail API (https://developers.google.com/gmail/api/guides).
JMAP will only get adoption IMO if there is buy-in from the email market giants. At this moment, I can't pinpoint what that benefit would be for Gmail et al.