How to thread emails in IOS if they are in different folders?

I am used Blackberry emails for around 10 years. the feature i miss the most is the threading feature even emails are in different folders whether the sent folder or with other folders including rules. However, for the sake to make it easier, we can disregard the sent folder and having threads for only the unread emails (all of unread under one folder). i think this is the problem is iPhone recognize the "unread" for the Inbox folder (unread) only. I believe if they let the unread folders includes all the unread, it will be threaded easily.