Так должно быть?
$contentMail .= "Content-type: multipart/alternative;";
$contentMail .= "boundary=\"KJDFKLAJSDLFJADLKJ\"\r\n\r\n";
$contentMail .= "--KJDFKLAJSDLFJADLKJ\r\n";
$contentMail .= "Content-Type: text/plain; charset=".$this->smtp_charset."\r\n";
$contentMail .= "Content-Transfer-Encoding: base64\r\n\r\n";
$contentMail .= chunk_split(base64_encode($plain));
$contentMail .= "\r\n";
$contentMail .= "--KJDFKLAJSDLFJADLKJ\r\n";
$contentMail .= "Content-Type: text/html; charset=".$this->smtp_charset."\r\n";
$contentMail .= "Content-Transfer-Encoding: base64\r\n\r\n";
$contentMail .= chunk_split(base64_encode($html));
$contentMail .= "\r\n";
$contentMail .= "--KJDFKLAJSDLFJADLKJ--\r\n";