In this case, I think that caused a problem for a different reason. Out of convenience (and this is something I will not do again), I sometimes start a new mail message by "replying" to an old one, then changing to my new subject line. I had assumed (without ever bothering to check) that mail threading is done based on subject line, and had not noticed that the mail headers contain the "In-Reply-To:" information that no doubt is used for connecting the mail threads.
Yes; in my case the gmane.org mail/NNTP gateway uses that to make its references headers, and Mozilla Thunderbird uses those headers to display and manipulate threads when I read the gmane.comp.lang.smalltalk.squeak.general newsgroup.
So in the future I will not use "reply to" unless actually replying to something.
Thanks again!
-C
-- Craig Latta www.netjam.org +31 6 2757 7177 (SMS ok) + 1 415 287 3547 (no SMS)