Outlook on the Web – Intermittent Inline Images and Attachment Thumbnails Failing (GetAttachmentThumbnail / HTTP 503)

We have been experiencing an organization-wide issue with Outlook on the web for approximately six months and I'm hoping someone from Microsoft or another Exchange Online administrator has encountered this and identified a resolution.

The issue is intermittent, affects multiple users across different geographic locations, and occurs regardless of browser, computer, or network.

Symptoms

Inline images embedded within emails intermittently fail to display.

Image attachment thumbnails frequently remain as generic gray placeholders instead of displaying the actual images.

Some thumbnails within the same email load correctly while others do not.

Refreshing the page (F5) causes a different, seemingly random selection of thumbnails to load or fail.

Simply scrolling down and back up can cause previously missing thumbnails to appear, while previously visible thumbnails may fail to load.

The issue can persist for hours or days before disappearing entirely.

Outlook may then work perfectly for several days before the failures return.

Previously received emails are also affected, including messages whose images displayed correctly in the past.

This is not limited to newly received messages or particular image formats.

When Outlook is functioning normally, the gray placeholders are replaced by the actual thumbnails almost instantaneously. During a failure period, many placeholders remain indefinitely.

Environment

Exchange Online / Microsoft 365

Multiple users across different countries

Both corporate and residential networks

Multiple browsers, including Chrome and Edge

No Outlook add-ins deployed

No consistent relationship to a particular sender, recipient, or attachment type

Classic Outlook displays the same images correctly

We have already ruled out individual browser installations, local network configurations, and individual user accounts.

Evidence from Edge Developer Tools

We captured the issue while it was actively occurring and identified numerous successful and failed requests to Microsoft's attachment-thumbnail service.

Attachment Thumbnail Endpoint

URL:

https://attachments.office.net/owa/[mailbox]/service.svc/s/GetAttachmentThumbnail

Failed Request Details

Request Method: GET

HTTP Status: 302 Found

Redirect Location: /owa/auth/errorFE.aspx?httpCode=503

OWA Result Type: ServerError

Response Origin: OwaNetCore

Proxy Backend Server Status: 302

Proxy Routing Correctness: 1

Technical Finding

Outlook on the web intermittently fails to retrieve attachment thumbnails. Instead of returning the requested image, Microsoft's attachment service returns an HTTP 302 redirect to an error page indicating 503 Service Unavailable.

Other thumbnail requests made during the same session successfully return HTTP 200 OK.

This behavior has been reproduced across multiple users, browsers, computers, and geographic locations, suggesting a failure within Microsoft's Outlook on the web attachment retrieval infrastructure.

Given that the failures are occurring at Microsoft's attachment endpoint, and successful and failed requests are being processed by the same Microsoft infrastructure, this does not appear to be a local browser, network, or standard Exchange Online policy issue.

We would appreciate a technical explanation or resolution rather than generic browser-cache troubleshooting, which does not address the organization-wide nature of the problem.

Has anyone encountered the same behavior and found a permanent solution?

Many thanks for any help,

Justin