Oh come on. The standard "if it's so easy why don't you just build it yourself and open-source it" is a meaningless reply. There are tons of things in the world

Oh come on. The standard "if it's so easy why don't you just build it yourself and open-source it" is a meaningless reply. There are tons of things in the world that are pretty easy to implement if you are provided the supporting systems and environment. If I had already put in the tons of hard work to build an email client I would write the threading piece the way I suggested above. But I haven't. My point above is that the algorithm probably isn't terribly difficult for someone to implement. I don't think the fact that I will never implement an email threading algorithm precludes me from making that assessment. The algorithm may be more complicated. It may not be. I doubt that it is significantly more complicated. I certainly wouldn't bet my first born on it, but then I doubt that you actually would either.