Building a Gmail Watchdog: Automated Email Monitoring with Google Apps Script

I needed a way to monitor critical emails that should arrive regularly however, when they do not, I need to be notified. Rather than paying for expensive monitoring services, I built my own solution using Google Apps Script that runs 24/7 in the cloud and sends professional alerts when expected emails go missing.

What was the mission?

I have several automated systems that send me important emails:

Daily email confirmations from lee@bythepowerofskeyskull.com
Server status reports with subjects starting with "Automated Email Check"
System health notifications that arrive every 15 minutes

If emails were not being delivered I would like to get a notification, but this needed to come from outside the company (I already have an internal one)

What I needed was a monitoring system that would:

Check Gmail every 15 minutes for expected emails
Stay completely silent when emails arrive normally
Send immediate alerts when emails don't arrive
Run automatically without any infrastructure on my end

Houston "we have a problem" Notification

This is a visual of the notification that is sent when the mail is not detected as per the script configuration, it needed to be very cleat and easy to read:

Core Monitoring Logic

Here's the heart of the monitoring system:

function checkForExpectedEmails() {
  // Calculate time range (last 15 minutes)
  const now = new Date();
  const checkTime = new Date(now.getTime() - (CONFIG.checkWindow * 60 * 1000));
  
  // Build Gmail search query
  const searchDate = Utilities.formatDate(checkTime, Session.getScriptTimeZone(), 'yyyy/MM/dd');
  const fullSearchQuery = `${CONFIG.searchQuery} after:${searchDate}`;
  
  // Search for emails matching criteria
  const threads = GmailApp.search(fullSearchQuery, 0, 10);
  
  // Only alert if NO emails found
  if (threads.length === 0) {
    console.log('No expected emails found - sending alert');
    sendAlert();
  } else {
    console.log(`Found ${threads.length} expected email(s) - no alert needed`);
  }
}

Configuration Setup

I made the system configurable so I could easily monitor different email types, this was looking for the email being sent into the Google Mail from a certain sender and with a certain subject.

const CONFIG = {
  // Email to send alerts to
  alertEmail: 'lee@bythepowerofskeyskull.com',
  
  // Search criteria for emails you're expecting
  searchQuery: 'from:manager@idobeleiveinfaries.co.uk subject:"Automated Email Check"',
  
  // Time window to check (in minutes)
  checkWindow: 15,
  
  // Alert details
  alertSubject: 'Email Alert: Automated Email Check not received',
  alertFromName: 'Email Monitor'
};

This searches for emails from a specific sender with subjects containing "Automated Email Check". Gmail automatically matches partial subjects, so it catches emails like:

"Automated Email Check - Daily Report"
"Automated Email Check: System Status"
"Automated Email Check (July 19)"

The target emails were as below, so this filtered worked well:

Professional Alert Emails

When something goes wrong, I want clear, actionable alerts. I built professional HTML email templates that look like enterprise monitoring systems:

function sendAlert() {
  const htmlBody = `
<!DOCTYPE html>
<html>
<body style="font-family: 'Segoe UI', sans-serif; margin: 0; padding: 0;">
  <div style="max-width: 600px; margin: 0 auto; background-color: #ffffff;">
    
    <!-- Alert Header -->
    <div style="background: linear-gradient(135deg, #ff6b6b, #ee5a24); color: white; padding: 30px 20px; text-align: center;">
      <div style="font-size: 48px; margin-bottom: 10px;">⚠️</div>
      <h1 style="margin: 0; font-size: 24px;">Email Monitoring Alert</h1>
      <p style="margin: 5px 0 0 0; opacity: 0.9;">Expected email not received</p>
    </div>
    
    <!-- Alert Details Table -->
    <div style="padding: 30px 20px;">
      <table style="width: 100%; border-collapse: collapse;">
        <tr>
          <td style="padding: 12px 15px; font-weight: 600;">Search Criteria:</td>
          <td style="padding: 12px 15px; font-family: monospace;">${CONFIG.searchQuery}</td>
        </tr>
        <tr>
          <td style="padding: 12px 15px; font-weight: 600;">Status:</td>
          <td style="padding: 12px 15px;">
            <span style="background-color: #dc3545; color: white; padding: 4px 12px; border-radius: 20px;">MISSING</span>
          </td>
        </tr>
      </table>
    </div>
    
  </div>
</body>
</html>
  `;

  GmailApp.sendEmail(CONFIG.alertEmail, CONFIG.alertSubject, textBody, { htmlBody: htmlBody });
}

These alerts include gradient headers, status badges, detailed tables, and recommended actions - much more professional than basic text emails.

Alert Strategy: Silence is Golden

I made a crucial design decision: the system only sends emails when there's a problem. There are no "everything is fine" notifications.

This prevents alert fatigue - when people receive too many routine notifications, they stop paying attention to alerts entirely. With this system:

Silence means everything is working
An email means immediate attention required

This approach ensures that when I do receive an email, I know it's genuinely important.

Step-by-Step Installation Guide

Here's how to set up the email monitoring system:

1. Create the Google Apps Script

Go to script.google.com
Click "New project"
Delete the defauly myFunction() code:
Paste the data (when requested from the link)
Save the project with a meaningful name like "Email Monitor"

2. Configure Your Settings

Update the "config" object at the top of the script:

Set alertEmail to where you want alerts sent
Set searchQuery to match your expected emails
Adjust checkWindow if you want different timing
Save the script (Ctrl+S)