What if we could use standard e-mail threading properties (reply-to and so on) to implement responses to older message? It would significantly improve experience of reading emails from people out of Delta who send answers for different threads to you; sometimes I have three different topics of different pace with one correspondent.
If Delta identify message in conversation, which is being replied to, then it should be shown in the style @bennis initially attached. (When it’s not previous message.)
If you want to reply to an arbitrary message in conversation. then your reply should properly refer to this message. That way people will get nice thread branch in traditional MUA as well will see chat-style reply in Delta.
When you just add a message in conversation it refers to the last message if time from
reply to last message is small [^1], and start new “thread” if the time is over threshold. The threshold should be customizable (even could be adaptive in future).
There also could be an icon if new thread will be started by your message (maybe with horizontal line). And if you want to continue previous thread you should select a previous message (last or another) for reply; alternatively the icon could be clickable just to continue last thread (remove the line) — last option need additional coding for usability, first option relies on previous features.
What to do with inline responses?
[^1]: Currently I usually just send a new mail from MUA if there was no chat activity with the person for an hour or two. And if counterparty is actively engages in the conversation, then I take advantage of Delta to speed things up.