Из комментов:
>A 'chat' client in the style of eg WhatsApp, but based on S/MIME over SMTP/IMAP, seems perfectly doable, and appropriate for most people's needs, with the obvious advantage of being supported by traditional email clients as a fallback.
>Additionally, message threading is the feature I most appreciate in a messaging system, but which is painfully lacking in most products (and no, Slack doesn't cut it). SMTP has built-in support for it.
>marcus_holmes 1 day ago [-]
>I'm old enough to remember when this was exactly how email was used. (but using a normal email client).
It was acceptable to send one-word email replies, and there were email chains of hundreds of emails (I was the guy who tended to "snip" them after 20 or replies).
>Now email seems to have taken over from where snail mail was: bills, newsletters, and formal communication. Chat is now the norm.
>Though I do notice a generational divide: one of my co-founders is in his 60's, and will phone randomly (which is now considered rude), another is a bit younger and prefers email to messaging, but will message to ask if it's OK to call. My younger colleagues send formal email replies, and use Whatsapp/Keybase for all other communication. I vetoed using Slack in the organisation completely ;)
>I wonder if in another 30 years, chat apps will be the formal channel, and something else will have taken over for just chatting.
https://news.ycombinator.com/item?id=21944875