[GNC] Why do people start new threads?

[Fred Bone]

On 13 December 2021 at 8:36, Chris Green said:

> On Sun, Dec 12, 2021 at 04:06:48PM -0800, Dave Hayes wrote:
> > On Sun, 12 Dec 2021 21:52:49 +0000
> > Chris Green <cl at isbd.net> wrote:
> > > On Sun, Dec 12, 2021 at 01:16:12PM -0800, Dave Hayes wrote:
> > > > On Fri, 10 Dec 2021 15:24:15 +0000
> > > > "Dr. David Kirkby" <drkirkby at kirkbymicrowave.co.uk> wrote:  
> > > > > Can't people reply to an existing thread, without creating another
> > > > > one?  
> > > > 
> > > > There are specific headers (e.g. References, In-Reply-To) that have
> > > > to be present for most mail clients to thread properly. The
> > > > exclusion of those headers is often a mail client issue and often
> > > > happens without the awareness of the responder. Thus, it may not be
> > > > their intention to do this. :)  
> > > 
> > > The Message-ID: is what should be used, that's the fundamental reason
> > > for having a Message-ID:.  Using anything else is second best.
> > 
> > That is always used and mostly present. However, there's nothing to
> > compare it to unless you at least have an "In-Reply-To" header.
> > Otherwise, how do you know a message has been replied to? 
> 
> Yes, of course, if someone's mail client doesn't put the Message-ID:
> there then there's not much one can do.  My mail client will thread
> using the Subject: if required but it's far from the 'right' way to do it.

Message-ID: is one of the few required headers. If the MUA fails to 
supply it then the first MTA should do so.

Threading relies on the In-Reply-To: which is supposed to contain the 
contents of Message-ID: from the message being replied to. If the MUA 
fails to create it then nothing else can.