Are there email clients that reduces friction/latency in communication?

Problem statement: I'v been thinking why don't we have "Slack" that uses plain old email as the protocol, enabling communication with everyone. Using outlook feels heavy. Pressing reply, scrolling through the conversation, not-so-fast-search, composing new email and yet again inputting those recipients has so much "friction".

Are there any email clients that are available? My wish list would be some email client that comes close to:

- Working offline (my mails don't have to be routed somewhere through "the cloud")

- Clever UI that groups email by recipients / threads / priority inbox.

- Can create my own "groups" / "rooms" (i.e "Developers"), with particular recipients. I'v visualized that when starting new conversation (i.e different subject) I kind of start some new thread.

- I don't have to press "reply all" all the time

- Search is fast.

- Exchange support?

That would greatly reduce friction in using email.

I'v found https://missiveapp.com - email + chat combined - neat. However it's not obvious how it works. I need to register to use it (okay). But after registration, I have to provide extensive permissions to my Gmail :( Googling for "mail like chat" brings up many matches... But not that I want my email to be exactly like, say, whatsapp.

Suggestions?