On Friday 22 April 2016 13:56:49 The Wanderer wrote: > Interesting. Do you have any evidence for the idea that it uses more > than just Message-ID? I can't prove that it doesn't, but I've never seen > anything that I recall to indicate that it does. It doesn't use MORE than the message ID. It doesn't use the message ID, so far as I can see, at all. It uses the subject header. That is why those using the Gmail interface munge threads so appallingly. Lisi