Too many security holes, discovered and undiscovered with JavaScript, not to mention JMTP.
Better yet,
Hashing the local part of the email address and retrofitting mail clients to do the mapping and rejection is the best way to delete unwanted and unsolicited emails.
Couple that with new decentralized lookup of sender’s PKI (as well as DKIM).
That should add years/decades to SMTP viability and usability.
There is no "JavaScript" in JMAP in the sense that it's supported by the protocol, and the specification explicitly warns against email clients interpreting JavaScript.
Potential and actual vulnerabilities explodes, like the chronic problems of Microsoft Macros.
The exploits writers would like to politely disagree. /s
It has it's own quirks, but you don't have the Google IMAP weirdness.
Anyhow, I am so happy I recently took the time to set up thunderbird for my gsuite account at work. I've been using thunderbird for a long time privately and the UX is just so much better than gmail, it really improved my quality of life during work hours.
Maybe if someone standardised such an extension they'd add support. Considering how ossified email is, they rightfully have little interest to do it alone.
I mostly use Apple Mail, which is acceptable, if not great. I guess I need to look at Thunderbird (which I haven't looked at since I've been using a Mac) and, maybe, Vivaldi.
AWS does industrial espionage, Google probably does too... Not that the encroaching reliance on Microsoft is much smarter...
Google is desperate to find a revenue tent-pole that even approaches the value of ads, and they've bet heavily on that tent-pole being Cloud. Any short-term gains they could acquire from cracking open a user's email would be dwarfed by the long-term losses from ruining their reputation as a reliable keeper of secrets, and they need that reputation to trade in the Cloud space at all.
But the nightmare that is IMAP has prevented me from ever succeeding in cleanly integrating the two. In fact, the general experience of using Microsoft and Google products together is quite frustrating. I get that they are competitors, but not acknowledging the multi-platform reality of modern work and forcing me to suffer because you can't be healthy adversaries, is really ridiculous and I resent both companies for their user-hostile practices.
Looking at you too, Apple.