Does gmail not use the In-Reply-To or References headers? Or is its use of them just brittle? (While googling for the names of these headers, I came across htt

Does gmail not use the In-Reply-To or References headers? Or is its use of them just brittle? (While googling for the names of these headers, I came across http://www.jwz.org/doc/threading.html http://www.jwz.org/doc/threading.html , which is an excellent guide to the difficulties of implementing threading in MUAs.)