Treat it in what way? If your talking about threading that uses the references header, not anything on the subject line by default.

Yes, it's about threading as shown in the following pictures.

I have edited the headers of the different replies and I have noticed that in the replies of Thunderbird there is a reference to the message ID of the original message such as : In-Reply-To: <55CC379F.6000907@free.fr> References: <55CC379F.6000907@free.fr> But this reference is not present in the french mail-client. There is no way to correct this apart from changing the client ! Many thanks for your reply and sorry for the noise.

There are hidden preferences for Threading that can be set with the config editor discussed here perhaps hat may assist your.

This "french mail-client" is that Thunderbird with a French translation and there is an issue with the translation? Or are you talking about another mail client there.

If the translation of Thunderbird to French adds an inappropriate space in the French localization, then we should file a bug to get the translation corrected.

1. Thank you very much. http://kb.mozillazine.org/Stop_threading_by_subject It is exactly what I was looking for. This solved my problem.

2. No the mail client is not Thunderbird with a french translation. It is a not well known french software. But during my investigations I have discovered that Hotmail has the same behavior : no reference in the reply to the message-ID of the original message.