SMTPMIME how to set encoding for the body

I am using IS 6.01 SP2, I send email to various users using the standart smtp service. I map a string to the body. Not attachment. Some users does not display fine the mail body : the Euro symbol and apostrophe are replaced by ?.
I can switch from default utf-8 to iso-8859-15 using the charset parameter, but the MIME is always encoded with base64.
Do I need to switch to Content-Transfer-Encoding: quoted-printable ? or 8bit ? How ? Do I need to build myself complete MIME part (createMimePart) ?
Or should I check for the config on the mail server (MS Exchange 5.5) ?