How to modify the character set

[yoxud]

I am trying to use the image as a smtp-relay to send emails with french ascii characters. The fench characters in the subject line are received correctly but all non us ascii characters in the body are removed. I tried to modify run.sh like so

if [ ! -z "${OVERWRITE_FROM}" ]; then
echo -e "/^From:.$/ REPLACE From: $OVERWRITE_FROM" > /etc/postfix/smtp_header_checks
echo "/^Content-Type:.$/ REPLACE Content-Type: charset=iso-8859-1" >> /etc/postfix/smtp_header_checks
postmap /etc/postfix/smtp_header_checks
postconf -e 'smtp_header_checks = regexp:/etc/postfix/smtp_header_checks'
echo "Setting configuration option OVERWRITE_FROM with value: ${OVERWRITE_FROM}"
fi
The content-type modification is appended to the smtp_header_checks file but the email is still being sent as us-ascii. Any clues on how to allow for non us-ascii characters in the body?