The people I know who don't use Slack yet are mostly older and mostly use Outlook. I think they'd be confused and irritating if we sent them emails with the frequency of chat.
Especially phone email notifications are not tuned to correctly handle instant messages.
I have often dreamed of a similar concept, but have never been able to work the UX out in my mind. Hopefully you have. Best of luck!
We're planning daily digest emails to group together multiple messages with individual reply buttons.
Showed it and explained it to my gran and she seemed to get it just fine. This is a woman that couldn't understand why her iPad wasn't charging until I explained it did actually need plugging into the wall. lol.
I think anyone would get confused and irritated by some weird confusion over email and Plum Mail. I think the next step is to ship v1 and see how users get on with it. I'll try to remember to report back what we learn.