> On Dec 16, 2022, at 12:04 PM, ic <li...@benappy.com> wrote:
> 
> Hi there,
> 
>> On 16 Dec 2022, at 17:13, William Herrin <b...@herrin.us 
>> <mailto:b...@herrin.us>> wrote:
>> 
>> Most email clients assume that a change to the subject line (other
>> than adding "Re:" to the front) indicates that the sender wants to
>> discuss a new topic related to but meaningly different from the last.
> 
> Although I generally agree that changing the Subject line without reason is 
> an annoyance, I didn’t notice any issue with it until I came across this 
> thread, which wasn’t broken in my mail client (Apple’s Mail.app).

As a user of Mail.app as well, it is not broken for me either.  However, reason 
being — Mail does not use just the subject to thread.  I used to nerd out about 
email (top + bottom posting, etc) so the details of how Apple Mail threads have 
been lost in my ADHD riddled brain, but — that’s why.

> 
> This led me to a few tests, and FWIW even Mutt seems happy with the Subject 
> changing and still threads the emails appropriately.
> 
> In my experience, threading is done by clients looking for the In-Reply-To: 
> header, not subject. Subject is a heuristic fallback, in case In-Reply-To is 
> absent.
> 
> Some email clients (although I don’t remember which ones) remove In-Reply-To: 
> when the Subject: is changed (that might go as far back as my Gnus Oort days).
> 

…. and now that I wrote the above email response, I think you’re right.  
In-Reply-To:  I believe, is how Mail.app does it. (And several others)

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