It looks like there is some kind of mixing like you say.
For the same paragraph, in the one I receive from my client (which displays OK on my screen) :
------_=_NextPart_003_01CFED64.5E0191AB
Content-Type: text/plain;
charset=”iso-8859-1”
Content-Transfer-Encoding: quoted-printable
Bonjour Nicolas,
=20
Suite =E0 notre discussion, pour ESS il s’agira donc de d=E9velopper une =
imagerie plut=F4t qu’une signature ou qu’un sceau, ou tag comme ce qui =
avait =E9t=E9 propos=E9. Comme discut=E9, l’image pourra =EAtre =
compos=E9e de plusieurs images ou d’un m=E9lange d’illustration et =
d’image, qui =E9voquent le ESS (rest of text)
It comes like this (I asked to send me the mail that appear with chinese to them) :
------_=_NextPart_001_01CFEE26.44842963
Content-Type: text/html;
charset="utf-8"
Content-Transfer-Encoding: base64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(with a lot of lines like this, as a big block)