When unread emails pile up to thousands or even tens of thousands, Gmail’s front-end UI becomes unusable: select-all freezes, pages stutter, and operations get cut off midway. The core of this method is bypassing Gmail’s front-end UI entirely and calling the Gmail API directly on Google’s backend via Google Apps Script.

Core Principle

The script follows a “paginated search → batch mark → sleep to avoid rate limits → loop” pattern. It executes server-side on Google’s infrastructure, forces the read-state refresh, and tracks its own progress in logs.

Full Steps

  1. Set up the environment: open script.google.com in your browser and create a new project from the top-left menu.

  2. Deploy the code: clear the default code and paste in the following script, which includes progress logging:

function markAllAsRead() {
  Logger.log("Started marking emails as read...");
  let totalProcessed = 0;

  // Search the first 100 unread threads each round
  let threads = GmailApp.search('is:unread', 0, 100);

  while (threads.length > 0) {
    GmailApp.markThreadsRead(threads); // mark batch as read
    totalProcessed += threads.length;
    Logger.log("Marked " + threads.length + " threads. Total so far: " + totalProcessed);

    Utilities.sleep(1000); // pause 1s to avoid API rate limits
    threads = GmailApp.search('is:unread', 0, 100); // fetch next batch
  }

  Logger.log("Done! Total threads marked as read: " + totalProcessed);
}

  1. Authorize and run: hit Save (the floppy icon) in the toolbar, then Run. On the first run, a permission prompt appears — pick your Google account and grant access (if warned about an “unverified app”, choose Advanced → Go to project).

  2. Watch the progress: after running, an Execution log panel opens at the bottom of the editor, showing batches being processed in real time until the final summary line prints.

Why This Method Holds Up

  • Immune to UI state: whatever your inbox layout is (categories, priority inbox, etc.) doesn’t affect the underlying API execution.
  • Rate-limit safe: the Utilities.sleep(1000) call keeps requests from being rejected by Google’s servers.
  • Stable at scale: the while loop plus 100-thread batches can chew through thousands or tens of thousands of backlogged emails without freezing your browser or timing out the script.

Note: the script marks every unread thread it finds as read. If you only want a subset, adjust the search query — e.g. 'is:unread older_than:1y' to only touch mail older than a year, or add a label filter.