Ronald,
the issue here is possibly the content-type and where the mail client gets this information from. Most clients get the content type from the header and some, like Gmail convert the content-type to UTF-8 see here for an explanation. Inspect the headers of the mails to review the content-type. An option could be to convert the special chars to their html entities.
Does the email allow for meta tags like : <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
We had that too, in our Mendix 5 project. Mendix made a fix to the email template module because Outlook misunderstood the content type settings where other email clients worked perfectly.
I don't know whether that fix has been ported to the Mendix 4 version too.