PHP使用PHPMailer发送OTP验证邮件失败的原因排查

我正在尝试给用户发送包含验证验证码（OTP）的邮件，确认SMTP配置完全正确（上周还成功用于发送带PDF附件的邮件），但当前发送OTP邮件失败，调试时未发现任何错误，请问还有哪些可能的原因？

<?php

use PHPMailer\PHPMailer\PHPMailer;

require_once(__DIR__ . '/sendEmails/vendor/phpmailer/phpmailer/src/Exception.php');
require_once(__DIR__ . '/sendEmails/vendor/phpmailer/phpmailer/src/PHPMailer.php');
require_once(__DIR__ . '/sendEmails/vendor/phpmailer/phpmailer/src/SMTP.php');


class VerificationCode
{
    public $smtpHost;
    public $sender;
    public $smtpPort;
    public $password;
    public $receiver;
    public $code;


    // some function to receive,send and port.

    public function _constructor($receiver) {
        $this->sender = "xxx@example.org";
        $this->password = "****";
        $this->smtpHost = "mail.example.com";
        $this->smtpPort = 587;

    }
    // function to send email().
    public function sendMail(){

        $mail= new PHPMailer();
        $mail->isSMTP();
        $mail->SMTPAuth = true;
        $mail->SMTPOptions = array(
            'ssl'=> array(
                'verify_peer'=>false,
                'verify_peer_name' => false,
                'allow_self_signed'=> true
            )
            );

            $mail->Host = $this->smtpHost;
            $mail->Port =$this->smtpPort;
            $mail->IsHTML(true);
            $mail->Username =$this->sender;
            $mail->Password= $this->password;
            $mail->Body=$this->getHTMLMessage();
            $mail->Subject = "Your verification code is {$this->code}";
            $mail->SetFrom($this->sender, "Verification Code");
            $mail->AddAddress($this->receiver);
            if($mail->send()) {
                echo "Mail Sent Successfully";
                exit;
            }
            echo "Failed to Send Mail";
    }

    // function to get html message from the email.
    public function getHTMLMessage() {
        $this->code=$this->getVerificationCode();
        $htmlMessage=<<<MSG
        <!DOCTYPE html>
        <html>
            <body>
                <h1>Your verification code is {$this->code}</h1>
                <p>Use this code to verify your account.</p>
            </body>
        </html>
        MSG;
        return $htmlMessage;
    }

    // get verificationCode.
    public function getVerificationCode() {
        return (int) substr(number_format(time() * rand(), 0,  '', ''), 0, 6);
    }

}

// instantiate VerificationCode and send email
$vc=new VerificationCode('gcira2023@outlook.com'); 
$vc->sendMail();

?>

• 构造函数语法错误：代码中构造函数写成了_constructor，PHP的正确构造函数名是__construct（两个下划线），这会导致类的smtpHost、sender、password等核心配置属性完全未初始化，直接引发SMTP连接失败，这是当前代码最致命的问题。
• 反垃圾邮件系统拦截：OTP邮件内容简洁且包含验证码，极易被收件方或邮件服务商的反垃圾系统判定为垃圾邮件，直接进入垃圾箱或被拦截，即使SMTP发送成功，收件人也无法收到。
• SMTP服务器临时限制：即使上周配置正常，邮件服务商可能临时调整了发送规则（如频率限制、单日发送量阈值），或发件IP被临时列入监控名单，导致发送失败。
• PHPMailer错误信息未捕获：当前代码仅通过$mail->send()的布尔值判断结果，未启用异常捕获机制，无法获取SMTP连接、认证或发送过程中的具体错误详情。
• 验证码生成逻辑潜在问题：getVerificationCode方法通过time() * rand()生成验证码，极端情况下可能生成不足6位的数字，导致邮件内容格式异常，触发邮件系统过滤规则。
• 收件箱安全策略限制：收件邮箱（如outlook.com）可能开启了严格的安全策略，陌生发件人的邮件被直接拦截，或收件邮箱存储空间已满。

• 修正构造函数：将_constructor改为__construct，同时初始化receiver属性：

  public function __construct($receiver) {
      $this->sender = "xxx@example.org";
      $this->password = "****";
      $this->smtpHost = "mail.example.com";
      $this->smtpPort = 587;
      $this->receiver = $receiver;
  }
  

• 启用PHPMailer异常捕获：修改sendMail方法，添加异常捕获以获取详细错误：

  public function sendMail(){
      try {
          $mail= new PHPMailer(true); // 传入true启用异常模式
          $mail->isSMTP();
          $mail->SMTPAuth = true;
          $mail->SMTPOptions = array(
              'ssl'=> array(
                  'verify_peer'=>false,
                  'verify_peer_name' => false,
                  'allow_self_signed'=> true
              )
          );
  
          $mail->Host = $this->smtpHost;
          $mail->Port =$this->smtpPort;
          $mail->IsHTML(true);
          $mail->Username =$this->sender;
          $mail->Password= $this->password;
          $mail->Body=$this->getHTMLMessage();
          $mail->Subject = "Your verification code is {$this->code}";
          $mail->SetFrom($this->sender, "Verification Code");
          $mail->AddAddress($this->receiver);
          $mail->send();
          echo "Mail Sent Successfully";
          exit;
      } catch (Exception $e) {
          echo "Failed to Send Mail: " . $mail->ErrorInfo;
      }
  }
  

• 优化验证码生成逻辑：确保生成稳定的6位数字验证码：

  public function getVerificationCode() {
      return str_pad(mt_rand(100000, 999999), 6, '0', STR_PAD_LEFT);
  }
  

• 排查邮件拦截情况：检查发件邮箱的退信记录、收件邮箱的垃圾箱；可更换收件邮箱测试，或联系邮件服务商确认是否存在发送限制。

内容的提问来源于stack exchange，提问作者MyLegend2020 Wilson