True, threading precedes conversations. But threading is not the same thing as Gmail-style conversations.
If you have a two messages that belong to one thread but that you put in two different folders, a thread-aware email client will still only show one message when you view it.
In a conversion-based email program however, there exists no such thing as an individual message. There are only whole conversations. When you view a 'single message', you always see the whole conversation regardless of the 'folder' it resides in. Basically, it always collects all the messages that belong to a thread even if you look at one single lonely message sitting in your inbox.
Persona seems to go a step further and not only collect threads into conversations, but also collect all conversations between a set of people into people-streams.
Okay. I understand what you meant now. I see this mostly as a consequence of tagging: since all the messages are in the same store, it makes sense to get messages from the same thread no matter the choice of tags you're viewing. I think it's true that Gmail was the first to do that.
If you have a two messages that belong to one thread but that you put in two different folders, a thread-aware email client will still only show one message when you view it.
In a conversion-based email program however, there exists no such thing as an individual message. There are only whole conversations. When you view a 'single message', you always see the whole conversation regardless of the 'folder' it resides in. Basically, it always collects all the messages that belong to a thread even if you look at one single lonely message sitting in your inbox.
Persona seems to go a step further and not only collect threads into conversations, but also collect all conversations between a set of people into people-streams.