There is a bug report about threaded view, basically headers of some threaded email can be different and so the app fails to recognize them as threaded mails. The example I know is (ironically) the bug updates from Harmattan's bug tracker. But it should at least work for some mails.... try what was said above (reply to a message, and your reply and the mail you replied will show up as threaded mail) and it should work. See this: https://harmattan-bugs.nokia.com/show_bug.cgi?id=837