The Content-Type of the sent-out emails was somehow part of the body instead of the header, so extended ASCII and intl. chars. looked cryptic. Line 158 changed to
Setting the visitor's/subscriber's email as sender (From) may be problematic in case the domain has an SPF / TXT record. It is better to use our own email address as both sender and recipient and to add our visitor's email as Reply-To.
To make the confirmation page look nice with extended ASCII or intl. chars. too, a new line was added between line 193 and 194.