使用S / MIME(PHP)发送带附件的电子邮件

$semi_rand = md5(time());   
$mime_boundary = "==Multipart_Boundary_x{$semi_rand}x";   
$fileatt = "pathtoattachment";
$headers = array();
$headers['From'] = $email_from;
$headers['Subject'] = $email_subject; 
$headers['MIME-Version'] = "1.0";
$headers['Content-Type'] = "multipart/mixed; boundary="{$mime_boundary}"";    

$file = fopen($fileatt,'rb');   
$data = fread($file,filesize($fileatt));
$data = chunk_split(base64_encode($data));     
fclose($file);   

//message part
$email_message = "This is a multi-part message in MIME format.nn" .   
                "--{$mime_boundary}n" .   
                "Content-Type:text/html; charset="UTF-8"n" .   
                "Content-Transfer-Encoding: 7bitnn
                                     Please find the file attachednn";   


//file part
$email_message .= "--{$mime_boundary}n" .   
                  "Content-Type: {$fileatt_type};n" .   
                  " name="{$fileatt_name}"n" .   
                  "Content-Transfer-Encoding: base64nn" .   
                 $data . "nn" .   
                  "--{$mime_boundary}--n";   

$mfile = fopen("msg.txt","w");
fwrite($mfile,$email_message);
fclose($mfile);


$key = file_get_contents("mailcert.cer");

$encrypt = openssl_pkcs7_encrypt("msg.txt","enc.txt",$key,$headers);
if($encrypt){
    $data = file_get_contents("enc.txt");
    $parts = explode("nn",$data,2);

    // Send mail
    $ok = mail($email_to,$email_subject,$parts[1],$parts[0]);
}

该脚本有效,电子邮件已发送,可以在Outlook中解密,但结果如下：

--==Multipart_Boundary_x6434b5a09f1f49c571a633802cd36772x

Content-Type:text/html; charset="UTF-8"
Content-Transfer-Encoding: 7bit

Please find the file attached

--==Multipart_Boundary_x6434b5a09f1f49c571a633802cd36772x
Content-Type: application/octet-stream;
 name="1327490599scrippie.txt"
Content-Transfer-Encoding: base64

JG9sZElwID0gIjE5NS40Ni4zOS43MyINCiRuZXdJcCA9ICIqIg0KDQojIEdldCBhbGwgb2JqZWN0
cyBhdCBJSVM6Ly9Mb2NhbGhvc3QvVzNTVkMNCiRpaXNPYmplY3RzID0gbmV3LW9iamVjdCBgDQog
ICAgU3lzdGVtLkRpcmVjdG9yeVNlcnZpY2VzLkRpcmVjdG9yeUVudHJ5KCJJSVM6Ly9Mb2NhbGhv
[etc....]

--==Multipart_Boundary_x6434b5a09f1f49c571a633802cd36772x--

是否有其他方式发送带附件的加密电子邮件？或者这个问题有解决方案吗？