PHPMailer多次发送后出现‘Could not connect to SMTP Host’错误求助

我使用PHPMailer实现用户提交请求后向审批人发送邮件的功能，首次发送邮件可正常送达，但多次发送后出现Could not connect to SMTP Host错误。相关代码如下：

$resAdd = sqlsrv_query($conn, $queAdd) or die(print_r(sqlsrv_errors()));

if ($resAdd) {

    // passing true in constructor enables exceptions in PHPMailer
    $mail = new PHPMailer(true);
    $emailBody = "This email is for testing only. Please ignore. Please check the link document. <a href = 'http://localhost/einv/v1/viewDet.php?variable=$refNum'><label>Link</label></a>";

        // Server settings
        $mail->SMTPDebug = 2; // for detailed debug output
        $mail->isSMTP();
        $mail->Host = 'smtp-mail.outlook.com';
        $mail->SMTPAuth = true;
        $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;
        $mail->Port = 25;
        $mail->SMTPOptions = array(
          'ssl' => array(
          'verify_peer' => false,
          'verify_peer_name' => false,
          'allow_self_signed' => true
          )
          );
   
        $mail->Username = 'support_einv@outlook.com'; 
        $mail->Password = 'abc134'; 

        // Sender and recipient settings
        $mail->setFrom('support_einv@outlook.com', 'e-Inv Notification', 0);
        $mail->addAddress('user@outlook.com');

        // Setting the email content
        $mail->IsHTML(true);
        $mail->Subject = "[FOR TESTING, PLEASE IGNORE] Pending Approval : $refNum";
        $mail->Body = html_entity_decode($msg_body);

        $mail->Body = $emailBody;
        $mail->Debugoutput = 'html';

        if(!$mail->send()) {
            echo 'Message could not be sent.';
            echo 'Mailer Error: ' . print_r(error_get_last(), true);
        } else {
            echo 'Message has been sent';   
            $mail->smtpClose();
        }
   
   echo "<script type=text/javascript>";
   echo "alert('Your request submitted successfully.'),location.href='../viewDet.php?variable=$refNum'";
   echo "</script>";
}

我已尝试以下解决方案但均无效：

• 检查防火墙
• 使用$mail->Host = gethostbyname("smtp-mail.outlook.com");
• 添加smtpClose()
• 启用php.ini中的extension=php_openssl.dll

请问还有哪些可能导致该错误的原因？

内容的提问来源于stack exchange，提问作者unknownUser24