Comment 1 for bug 671746

Some more food for thought:

Mailody has somewhat unique behaviour and sorts by youngest child:
http://www.jwz.org/doc/threading.html

There are multiple pseudo-standards to relate messages:

References:
In-Reply-To:
Thread-*: (written by Microsoft products)

We should show the number of messages beside a thread.

One thread should be treated as having the date of the latest message in it.