solved
After some time, research and trial and error, i think i figured what's wrong.
edit edit //include/pear/Mail/mime.php, and replace ISO-8859-1 by UTF-8
around line 187
'html_charset' => 'UTF-8',
'text_charset' => 'UTF-8',
'head_charset' => 'UTF-8'
I guess because outlook compatibility all outgoing email is converted to ISO-8859-1.
I'm using v1.6rc5.