git-send-email: threaded view issue with Thunderbird

Hi,

Sometimes, actually most of the time, git-send-email _seems_ to not work properly with Thunderbird: for example if I send a patchset made up by 4 single patchs, then Thunderbird show me this patchset 'broken' that is [PATCH 2/4] is not shown as a reply to [PATCH 1/4].

For example I can see:

[PATCH 0/4]
       [PATCH 1/4]
[PATCH 2/4]
[PATCH 3/4]
       [PATCH 4/4]

instead of

[PATCH 0/4]
       [PATCH 1/4]
          [PATCH 2/4]
             [PATCH 3/4]
                [PATCH 4/4]

The strange thing is that all email headers are correct.

I recently notice that someone has sent a patchset using git-send-email and I can see it 'broken' as well. The subject of the first email of the set is

[PATCH 0/7] C cleanup series, mostly nasty shadowing

Does anybody have an idea on what's going wrong ?

thanks,

Francis