Auto-Respond to Gmail Emails with Google Apps Script

Whether you need an out-of-office reply, an automated acknowledgement for form submissions, or a keyword-triggered response, Apps Script gives you full control over Gmail auto-replies.

Basic auto-reply to unread emails

function autoReplyToUnread() {
  const query = 'is:unread subject:"Support Request"';
  const threads = GmailApp.search(query);

  threads.forEach(thread => {
    const messages = thread.getMessages();
    const lastMessage = messages[messages.length - 1];

    // Don't reply to yourself
    if (lastMessage.getFrom().includes(Session.getActiveUser().getEmail())) return;

    lastMessage.reply(
      'Thank you for reaching out! We have received your support request and will respond within 24 hours.\n\nBest regards,\nThe Support Team'
    );

    thread.markRead();
    Logger.log('Auto-replied to: ' + lastMessage.getSubject());
  });
}

Reply with a personalised HTML email

function autoReplyHtml() {
  const query = 'is:unread label:enquiries';
  const threads = GmailApp.search(query);

  threads.forEach(thread => {
    const lastMessage = thread.getMessages().pop();
    const senderName = lastMessage.getFrom().split('<')[0].trim() || 'there';

    const htmlBody = `
      <p>Hi ${senderName},</p>
      <p>Thank you for your enquiry. We've received your message and our team will be in touch shortly.</p>
      <p>In the meantime, you might find these resources helpful:</p>
      <ul>
        <li><a href="https://example.com/faq">Frequently Asked Questions</a></li>
        <li><a href="https://example.com/docs">Documentation</a></li>
      </ul>
      <p>Best regards,<br>The Team</p>
    `;

    lastMessage.reply('', { htmlBody });
    thread.markRead();
    thread.addLabel(GmailApp.getUserLabelByName('auto-replied') || GmailApp.createLabel('auto-replied'));
  });
}

Avoid duplicate replies with label tracking

function smartAutoReply() {
  const REPLIED_LABEL = 'auto-replied';
  const query = `is:unread subject:"Order Confirmation" -label:${REPLIED_LABEL}`;

  let label = GmailApp.getUserLabelByName(REPLIED_LABEL);
  if (!label) label = GmailApp.createLabel(REPLIED_LABEL);

  const threads = GmailApp.search(query);

  threads.forEach(thread => {
    const lastMessage = thread.getMessages().pop();

    lastMessage.reply('Your order has been received and is being processed. Thank you!');

    thread.addLabel(label);
    thread.markRead();
    Logger.log('Replied to thread: ' + thread.getFirstMessageSubject());
  });
}

Log auto-replies to a Google Sheet

function autoReplyAndLog() {
  const sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();

  if (sheet.getLastRow() === 0) {
    sheet.appendRow(['Timestamp', 'From', 'Subject', 'Status']);
  }

  const threads = GmailApp.search('is:unread subject:"Demo Request" -label:replied');
  const label = GmailApp.getUserLabelByName('replied') || GmailApp.createLabel('replied');

  threads.forEach(thread => {
    const msg = thread.getMessages().pop();
    const from = msg.getFrom();
    const subject = msg.getSubject();

    try {
      msg.reply('Thanks for requesting a demo! We\'ll be in touch within 1 business day to schedule a time.');
      thread.addLabel(label);
      thread.markRead();
      sheet.appendRow([new Date(), from, subject, 'Replied']);
    } catch (e) {
      sheet.appendRow([new Date(), from, subject, 'Error: ' + e.message]);
    }
  });
}

Run every 15 minutes with a trigger

function createAutoReplyTrigger() {
  ScriptApp.newTrigger('smartAutoReply')
    .timeBased()
    .everyMinutes(15)
    .create();
}

Tips

Always use a label to track replied threads — without it, you'll send multiple replies to the same email.
GmailApp.search() returns a maximum of 500 threads. If you expect high volume, process in smaller batches.
lastMessage.getFrom() returns a string like "Name <[email protected]>" — parse it carefully when extracting the name.
Be cautious with overly broad search queries. Test in a dry-run mode (log without replying) before activating the trigger.