Did the warrior send an e-mail with PHP+SMTP? Please point out the solution

Did the warrior send an e-mail with PHP+SMTP? Please give me a hint.
My server is loaded with SendMail, Mail can send, test, QQ third time to receive is in the spam mail? 163 not receiving?
May I ask why?

$to = "[email protected]";
$subject = "Test mail";
$message = "hello! A simple email message. ";
$from = "[email protected]";
$headers = "From: $from";
Mail ($to, $subject, $message, $headers);
echo "Mail Sent.";
?>

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can receive. Note that the letter is not a problem, may be intercepted by 163. Multi-Test.
------Solution--------------------
Use the system comes with the SendMail service all this, will have the suspicion of spam. Preferably using a third-party SMTP to send, many open-source libraries on the Web

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