Message threading: reply message and In-Reply-To/References

[Kurt Hackenberg]

On 2020-09-23 03:22, Francesco Ariis wrote:

> Il 23 settembre 2020 alle 13:52 Kevin Shell ha scritto:
>> In mail/news discussion,
>> some reply messages have no In-Reply-To/References headers,
>> what cause reply messages don't have In-Reply-To/References headers?
> 
> A possible explanation: the sender clicked «New message» and copy/pasted
> the Subject instead of clicking «Reply».


Another possibility is that the sender's mail reader just doesn't do 
that. Those two headers are optional, not required; some mail readers 
just don't create them in replies.