I apologize if this appears off the direct SL topic; however, please 
bear with me.

In a postscript comment and question to a recent email, as well as from 
responses to a previous discourse ("thread"), it appears that MUAs are 
highly inconsistent with respect to threads.  Irrespective of whatever 
RFCs that may exist on this matter, the reality is what is in the 
production MUAs.

Here is a suggestion.  At this epoch, Mozilla Thunderbird exists for 
essentially all workstation (end user interface computer) platforms and 
environments.  Thunderbird accepts plug-ins and extensions.  Can an 
extension be written for Thunderbird that properly handles threading?

Such an extension would eliminate/address issues such as:

Users of badly-threaded MUAs like mine (M$ OutLook) would see a new
thread because they go STRICTLY by the text of the subject line.
Users of well-threaded MUAs would see the new post in the old thread,
with a subject change.
It's a situation where you're damned if you do, and double-damned if you
don't.

End quote.

The only case under which my scheme could not work is when the 
organization/employer insists on a particular MUA (e.g., a MS product) 
that does not properly respect threading.

The caveat is once this is done, it would need to be maintained as 
Thunderbird evolves.

Again, I apologize for continuing this subject, but establishing a clean 
conforming standard avoids the sorts of discord that a well intended 
question caused.

Yasha Karant