[Andrew_nenakhov]

Email is just a messaging service. Whatever messaging service you use, the core is the same: people send you messages you need to act upon. Chat apps are no different from email in this regard, they just mask the problem of a non-zero inbox by merging all messages from one sender into a single thread. This way, important things you'd want to keep unread in email sinks into oblivion until you forget about it. (And feel good like you did everything you had to)

[combatentropy]

You just made me realize that email would look better if it grouped messages by sender instead of just by subject.

It was a huge step forward for me when I switched from Outlook to GMail. Outlook originally did not group emails at all. Each message was a line item on its own, sorted by date sent. So even individual replies back and forth were scattered across my Inbox. Then GMail grouped all replies into a single line item, as a "thread" or "conversation", which I could then expand. Eventually Outlook followed suit, although at first imperfectly. It grouped by Subject instead of the more accurate email header, Message-ID. (Actually I believe this technique predates GMail, at least in one text-based email reader, Mutt.)

I think email should take this one step further, and group messages first of all by sender, then by thread.

  sender (or recipient group for emails with more than one recipient, like SMS clients do)
  |- thread
     |- message

This would help with companies that send you many messages, whether it's spam or just receipts and other notices. But it would still not be bad for emails from individuals too. Even though someone may write you many emails about many different things, each with their own Subject, it still makes sense to group them. That mimics real life, where a real person's many conversations that they have with you are still visually grouped into one person, one organic body, in your mind.

Facebook would also be better this way. Collapse posts by poster, so that people who post 5 times a day don't take up more of your news feed than those who post just once every now and then.

[Andrew_nenakhov]

I was a heavy email user before Gmail advent, and was always using folders in my email clients to do just that. For me it turned out that most emails work better the gmail way. I'd benefit from a simple option to group all messages from/to a specific sender into one thread though