PHP と PHPMailer を使用して HTML 電子メールを確実に送信するにはどうすればよいですか?

composer require phpmailer/phpmailer

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;
use PHPMailer\PHPMailer\Exception;

$mail = new PHPMailer(true);

try {
    // 设置服务器的信息
    $mail->isSMTP();
    $mail->Host       = 'smtp.example.com';
    $mail->SMTPAuth   = true;
    $mail->Username   = 'username@example.com';
    $mail->Password   = 'password';
    $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;
    $mail->Port       = 587;

    // 设置发件人
    $mail->setFrom('from@example.com', 'From Name');

    // 添加收件人
    $mail->addAddress('to@example.com', 'To Name');

    // 设置主题和内容
    $mail->isHTML(true);  // Set email format to HTML
    $mail->Subject = 'Test HTML email';
    $mail->Body    = '<h1>Hello World!</h1><p>This is an <b>HTML</b> email.</p>';

    // 发送邮件
    $mail->send();
    echo 'Message has been sent using PHPMailer';
} catch (Exception $e) {
    echo 'Message could not be sent. Error: ', $mail->ErrorInfo;
}