JMAP is great but is not nearly enough. it would still need some improvements... on the protocols and on the available servers. I experienced slowdowns with Cyrus (which is the most advanced implementation I believe) on complex requests.
Also, the great idea behind JMAP is to chain requests so a series of tasks is performed on the server and the client then get the end result. This works great for the majority of cases but the replacement syntax (to replace results from a previous request in a following request) is not powerful enough in some cases (when trying to reuse complex responses in follow-up requests)
The other issue is that there is no way to easily groups e-mails from a given contact. There are JMAP extensions for CardDAV and CalDAV that can be of some use but it's difficult to bridge them with the Email spec and get all e-mails from a given contact for instance.
Last, it lacks editing features like appending a custom header to existing e-mails. The fact that JMAP abstract away all of the MIME format is great, but makes it difficult to just add a single header to an email.
Also, the great idea behind JMAP is to chain requests so a series of tasks is performed on the server and the client then get the end result. This works great for the majority of cases but the replacement syntax (to replace results from a previous request in a following request) is not powerful enough in some cases (when trying to reuse complex responses in follow-up requests)
The other issue is that there is no way to easily groups e-mails from a given contact. There are JMAP extensions for CardDAV and CalDAV that can be of some use but it's difficult to bridge them with the Email spec and get all e-mails from a given contact for instance.
Last, it lacks editing features like appending a custom header to existing e-mails. The fact that JMAP abstract away all of the MIME format is great, but makes it difficult to just add a single header to an email.