Microsoft is making an important change to inline images in Outlook

Microsoft has been trying really hard to get customers to switch the new Outlook experience, but it is facing stiff resistance from customers who think that the app is "hot garbage". Now, the company is preparing to make a change that does not only impact the new Outlook for Windows, but also the web implementation.

Basically, Microsoft is changing how inline images are represented in Outlook, which impacts developers who have built add-ins to process images in the service. Currently, an inline image's HTML contains an attachment ID. Developers can use this HTML code to extract the URL ID for comparison with the attachment ID, and utilize the URL to retrieve the Base64 value of the inline image.

However, moving forward, Microsoft is modifying this implementation to get rid of the attachment ID in favor of the content ID, which is a breaking change for certain Outlook add-ins which rely on parsing the former ID.

The rationale for this change is that Microsoft is transitioning to a fetch call to get inline images that includes the token in the request header, rather than using a GET request. Interestingly, this is a security improvement that is already utilized by classic Outlook for Windows, which means that the latest update only applies to the new Outlook for Windows and the web experience.

Microsoft has encouraged developers to update their content parsing logic to accommodate this change and to subsequently test their add-ins. The platform update will begin rolling out to production users starting from November 15, so it's crucial to modify your parsing logic before that date. The good news is that end users are not expected to be impacted by this change, unless they leverage add-ins which have not been updated by their respective developers to manage the HTML modifications to the attachment IDs.

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Posted Wednesday 8 October 2025 at 3:45 am AEST (my time).

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