[basemi]

If I read it right, it's built on Gmail API

Are there any other provider agnostic tools with similar capabilities?

[szszrk]

I guess JMAP was created to also deal with this. I'm not sure how far are we in implementation on clients side.

https://jmap.io/spec.html

[MangoToupe]

I can't imagine google ever supporting something that useful.

[fastasucan]

https://github.com/pimalaya/himalaya