OK, I tried UTF-8 charset, still reply-type. Then tried a bunch of other changes, finally keeping original iso-8859-1 and using quoted-printable seems to do it, thanks to my POP3 doing X-MIME-Autoconverted I can still read French accents I sent to myself. I will see what others receive,hopefully all POP3 server are like mine smart enough...
Content-Type: text/plain; charset=iso-8859-1
X-Spam-Status: No, score=0.0 required=5.0 tests=none autolearn=disabled version=3.2.0
X-Spam-Checker-Version: SpamAssassin 3.2.0 (2007-05-01) on mx-transit7.nfrance.com
X-MIME-Autoconverted: from quoted-printable to 8bit by albane.nfrance.com id q97CgKwn046669
I wanted to try that for a while, but not sure it's the right solution either reading http://mailformat.dan.info/config/oex.html:
The Plain Text Settings box determines some formatting for plain text messages. (I hope you chose to use plain text instead of HTML!) You can choose MIME or UUEncode encoding; this determines how attachments are coded. UUEncode is really archaic now, so MIME is the better choice. However, within the MIME section, you should pick "None" for the encoding choice. If you do this, you'll technically be breaking the standard if you use any characters outside the 7-bit US-ASCII range (e.g., if you include an accented letter), but choosing "Quoted Printable" (which would encode such characters and bring the message into compliance with the standards) unfortunately causes Outlook Express to violate another standard: that of line length. The "Automatically wrap text" item gets grayed out if "Quoted Printable" is used, and hard linebreaks are inserted only at the end of paragraphs, causing all sorts of display problems for other mail programs and Web-based mailing list archives. I used to think that this line-length problem happened in the plain-text versions of Outlook Express messages sent in HTML form, and went away when you picked plain text instead (yet another reason to prefer plain text over HTML), but it turns out that it's actually the "quoted printable" setting (which is by default on for HTML messages but off for plain text messages) that causes the problem.
So maybe like suggested Gavin to me, the best way would be to turn to Outlook 2010 that I have already installed, thanks to the MS license, no hurry...