We’d like to inform you about an update to how inline images are represented in the HTML body of Outlook emails in Outlook on the web and the new Outlook for Windows. This change may impact how your add-in identifies and processes inline images when using the attachment ID.

What’s changing?

Previously, we represented inline images in the HTML mail body with a URL containing the attachment ID as a query parameter. Developers could parse the HTML, extract the ID from the URL, and match it with the attachment ID they already had.

Going forward, the URL for inline images will no longer contain the attachment ID. Instead, the image will be referenced using a “blob” URL, which doesn’t include the identifier. As a result, your current parsing logic won’t be able to correlate the inline image in the mail body with the attachment ID.

Where is this changing?

This update applies to Outlook on the web and the new Outlook on Windows. Other Outlook platforms will continue to operate as they do today.

Why is this changing?

As part of our ongoing security efforts, we’re updating how inline images are loaded. Previously, images were retrieved via a GET request with the token embedded in the URL. We’re transitioning to a fetch call that includes the token in the request header. If you parse the attachment ID from the HTML editor to identify where the inline image appears in the message body, this change could impact you.

Solution

To restore inline image correlation, we are introducing a new attribute in the <img> tag called custom-id. The custom-id attribute will contain the actual attachment ID of the image. This allows you to continue to match the inline image in the message body with the attachment ID you have.

Action required

• Update your parsing logic Instead of extracting the attachment ID from the URL, parse the HTML message body and retrieve the value from the custom-id attribute of the <img> tag.
• Test your add-in Ensure your add-in correctly identifies and processes inline images using the new custom-id attribute.

Timeline

These changes will reach all production users by October 31, 2025. Starting November 28, 2025, deprecation of the id attribute will begin. Please make the necessary changes before then to avoid regressions to your add-in experience.