PHP和PHPMAILER：如何实现邮件发送的防垃圾邮件功能？

$recipient = 'xxx@example.com';
list($username, $domain) = explode('@', $recipient);

require 'phpmailer/PHPMailer.php';
require 'phpmailer/SMTP.php';

$mail = new PHPMailerPHPMailerPHPMailer();

$mail->isSMTP();
$mail->Host = 'smtp.example.com';
$mail->SMTPAuth = true;
$mail->Username = 'your-email@example.com';
$mail->Password = 'your-email-password';
$mail->SMTPSecure = 'tls';
$mail->Port = 587;

$mail->setFrom('from@example.com', 'Sender Name');
$mail->addAddress('to@example.com', 'Recipient Name');
$mail->Subject = 'Subject of Mail';
$mail->Body = 'This is the body of the email.';

if($mail->send()) {
    echo '邮件发送成功';
} else {
    echo '邮件发送失败：' . $mail->ErrorInfo;
}

// 反向DNS查找
$ip = gethostbyname($domain);
$hostname = gethostbyaddr($ip);

if ($ip !== $hostname) {
    // IP与域名不匹配，可能是垃圾邮件
    exit('The email could not be sent.');
}

// 检查邮件服务器的SPF记录
$result = dns_get_record($domain, DNS_TXT);
$hasValidSPF = false;

foreach ($result as $record) {
    if (strpos($record['txt'], 'v=spf1') !== false) {
        $hasValidSPF = true;
        break;
    }
}

if (!$hasValidSPF) {
    // 没有有效的SPF记录，可能是垃圾邮件
    exit('The email could not be sent.');
}

// 检查邮件服务器是否为匿名代理
$ip = gethostbyname($domain);
$hostname = gethostbyaddr($ip);

$proxyCheckUrl = 'https://example.com/proxy-check.php?ip=' . $ip;
$response = file_get_contents($proxyCheckUrl);

if ($response !== 'OK') {
    // 邮件服务器可能为匿名代理，可能是垃圾邮件
    exit('The email could not be sent.');
}