Hi, in the second mail-pair you find: On Tue, 7 Aug 2007 11:33:29 +0700 N.Novozhilov wrote: [1. msg] > Message-Id: <20070802163553.f79e565b.nna at ntrlab.ru> [2. msg] > References: <20070802163553.f79e565b.nna at ntrlab.ru> So the second has a reference to the first, and is threaded. The first mail-pair does not have such headers. --