lib/classes/Swift/Mime/MimePart.php
protected function convertString($string)
    {
        $charset = strtolower($this->getCharset());
        if (!in_array($charset, ['utf-8', 'iso-8859-1', 'iso-8859-15', ''])) {
            return mb_convert_encoding($string, $charset, 'utf-8');
        }

        return $string;
    }
protected function convertString($string)
    {
        $charset = strtolower($this->getCharset());
        if (!in_array($charset, ['utf-8', 'iso-8859-1', 'iso-8859-15', ''])) {
            return mb_convert_encoding($string, $charset, 'utf-8');
        }

        return $string;
    }
protected function convertString ( $ string ) { $ charset = strtolower ( $ this -> getCharset ( ) ) ; if ( ! in_array ( $ charset , [ 'utf-8' , 'iso-8859-1' , 'iso-8859-15' , '' ] ) ) { return mb_convert_encoding ( $ string , $ charset , 'utf-8' ) ; } return $ string ; }
Encode charset when charset is not utf-8
Encode charset when charset is not utf - 8