I have been testing out the plugin and noticed that after each response, the subject line adds "Re:" to the subject line and if the message goes on for a while with a lot of responses, the subject line is going to get very very long. It would be nice to remove all the extra "Re:". It's not a huge deal for me because I can hide it in CSS.
Another thought is to be able to set color of each user in a thread, maybe add a number to the "section.message-content" div to be able to set a color for the box. For instance, if there are 3 users in a conversation, the div would be labeled "section.message-content.user1" for the first user to send an email, "section.message-content.user2" for the second user that responds, and "section.message-content.user3" for the third user that responds. That way I can set different background colors in css for each user. I think that would make it easier to scroll through the conversation.
I hope that all makes sense.