> This seems somehow wild to me. Why don't other email apps use the appropriate header fields? Why do they even try to use the subject field? Why do they insist

> This seems somehow wild to me. Why don't other email apps use the appropriate header fields? Why do they even try to use the subject field? Why do they insist on quoting the whole thread text below the message body? Why not do all this the proper way? This all started with Microsoft Outlook back in the '90s. At that time, most email clients, even the GUI ones did handle threading using the References and In-Reply-To headers (even Outlook Express oddly enough). IIRC, the online webmail clients took the same approach that Outlook did in terms of message threading and quoting. As they became more popular and desktop clients fell by the wayside, the Outlook convention took over. FWIW, Thunderbird is one GUI based email client that also properly displays threaded messages. More recently, email clients have introduced the conversation view where they can display a thread as a conversation by only showing the parts of the series of messages before the quoted original message. This has an unfortunate side effect of essentially not displaying any message where someone decides to trim quoted material and post their responses inline. In a client that expects top-posted responses, an inline response shows up as a blank message (making the recipient think that you sent a blank email by mistake). But I do find it interesting that people want to treat email like a synchronous chat with a single thread of messages, but people want to have threaded conversations in chat clients like Slack.