I'd never heard about it and read their pitch on the site. As far as I can see the main point is that it has a fancy spam filter hat blocks everything by default untill you allow senders, and it has some proconfigured folders for different kinds of email like gmail does.
That looks fine but honestly I find that all email services and clients do the job now (Outlook, Thunderbird, Gmail all allow me to handle my mail the way I want), and it sounds like "Hey" won't even allow me to use any client I want ?
I think at this point email is a really mature (if flawed and limited) technology and noone is going to revolutionize the way we handle emails without breaking compatibility and creating a brand new protocol. To me the most exciting technology in that area was Google Wave, but it went the way of all Google messenging apps unfortunately. I guess in a way Slack, Teams, Discord, Hipchat and Mattermost are spiritual successors to Wave, but they won't ever replace emails because there's no unified protocol underneath.
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