为什么sendmail（ubuntu）+邮件功能（php）发送到SPAM文件夹（gmail）

Question

此代码用于发送邮件但不幸的是邮件转到垃圾邮件文件夹（仅限gmail）。 php代码是

$from = "Test <testFromEmail@gmail.com>"; 
 $headers  = "From: $from \n";
    $headers .= "Cc: $from\n"; 
    $headers .= "X-Sender: $from \n";
    $headers .= 'X-Mailer: PHP/' . phpversion();
    $headers .= "X-Priority: 1\n"; // Urgent message!
    $headers .= "Return-Path: $from \n"; // Return path for errors
    $headers .= "MIME-Version: 1.0" . "\r\n";
    $headers .= "Content-type:text/html;charset=UTF-8" . "\r\n";

$to      = 'TestToEmail@gmail.com';
$subject = 'subject';
$message = 'content';

if ( mail($to,$subject,$message,$headers) ) {
   echo "The email has been sent!";
   } else {
   echo "The email has failed!";
}

我是否需要添加或修改任何缺失的功能来解决此问题