PHP mailer 如何保存文本格式

下面的代码在发送到GMAIL，YAHOO，HOTMAIL时保留邮件格式，但在通过Outlook或Mozilla Thunderbird访问时相反，显示空消息。

我尝试了N2blr，但它给出了正确的结果。

知道吗？

$fp = fopen($attachment_temp, "rb");
$file = fread($fp, $attachment_size);
$file = chunk_split(base64_encode($file));
$num = md5(time());
//Normal headers
$headers  = "From: ".$namefrom."<".$mailhost.">'r'n";
$headers .="Reply-To: ".$emailfrom."'r'n";
$headers  .= "MIME-Version: 1.0'r'n";
$headers  .= "Content-Type: multipart/mixed; ";
$headers  .= "boundary=".$num."'r'n";
$headers  .= "--$num'r'n";
// This two steps to help avoid spam
$headers .= "Message-ID: <".gettimeofday()."   TheSystem@".$_SERVER['SERVER_NAME'].">'r'n";
$headers .= "X-Mailer: PHP v".phpversion()."'r'n";
// With message
$headers .= "Content-Type: text/html; charset=iso-8859-1'r'n";
$headers .= "Content-Transfer-Encoding: 8bit'r'n";
$headers .= "".$message."'n";
$headers .= "--".$num."'n";
// Attachment headers

$headers  .= "Content-Type:".$attachment_type." ";
$headers  .= "name='"".$attachment_name."'"r'n";
$headers  .= "Content-Transfer-Encoding: base64'r'n";
$headers  .= "Content-Disposition: attachment; ";
$headers  .= "filename='"".$attachment_name."'"'r'n'n";
$headers  .= "".$file."'r'n";
$headers  .= "--".$num."--";