Outlook Add-in w/ OnMessageSend - There was a problem sending this message because one or more attachments failed to upload. Please try again

[zacccypoo]

Your Environment

• Platform [PC desktop, Mac, iOS, Office on the web]: PC Desktop
• Host [Excel, Word, PowerPoint, etc.]: Outlook
• Office version number: 20260529014.16
• Operating System: Windows
• Browser (if using Office on the web): N/A

Expected behavior

An email should send using the native send without error

Current behavior

Customer using our Outlook Add-in that includes OnMessageSend in the manifest is consistently seeing emails fail to send, leaving the item stuck in the drafts folder.

After ~2 minutes a popup says "Couldn't Send this Message". Upon clicking more details the banner says

There was a problem sending this message because one or more attachments failed to upload. Please try again

The email has no attachments

Seems to be the exact same issue as #6708 (comment) however I was asked to make a separate ticket

The Email fails to send with the following errors

Steps to reproduce

1. Email with a long thread
2. User replies to email with basic, short text response
3. User presses the native send button
4. Item changes to have state [QUEUED]
5. Item remains in [QUEUED] for about a minute or so
6. Dialog appears with failed message notice
7. Item remains in the draft folders with status [DRAFT]

Link to live example(s)

N/A

Context

Customers are pissed because they dont realize that their emails dont send and are stuck in the draft folder

Useful logs

I followed these instructions https://github.com/OfficeDev/office-js/wiki/Collecting-log-files-for-Outlook-Add%E2%80%90ins-issues#new-outlook-on-windows

Here is the Diagnostic code - 7drFBA
And another customer's rWd7fA

I have downloaded a HAR file to share please provide instructions if you have any of how to do so securely.

Screenshots from the network tab

I tried to look at things that seemed suspicious from the network tab.

GetComplianceConfiguration took 1.5minutes and then failed without a response

There was a 403 on admin.microsoft.com/centrohost

Observations

• The thread was very long but there were no attachments
• There were however images in the signatures from recipients. These images, were all whited out, as if they failed to render, possibly due to IT Security Policies I am not sure..
• The email is sent successfully from the Outlook Web Client (chrome)`
• The email is sent successfully from Outlook Desktop when the recipients are internal
  • Our add-in OnMessageSend behaves differently for internal vs external. For external:
    • Adds a tracking <img /> pixel to at the top of the body
    • Replace all links in the thread with a custom tracking link by iterating over the anchor tags and calling setAttribute('href', newUrl)
    • Finally calls Office.context.mailbox.item.body.setAsync with the altered body with coercionType: Office.CoercionType.Html
    • Adds a custom email header via Office.context.mailbox.item.internetHeaders.setAsync
    • Note: everything is wrapped in try/catch and we are not seeing any failures during these OfficeJS calls. It appears as if our OnMessageSend successfully completes

[dmitriikashin-outreach]

We are seeing the same failure with another customer on New Outlook for Windows, and our log analysis points to a consistent root cause. Sharing in case it helps triage #6780 / #6708.

Brief summary

• Add-in declares OnMessageSend. User replies to a long thread with a short text response; the message has no file attachments.
• After ~2 minutes the send fails with "There was a problem sending this message because one or more attachments failed to upload. Please try again," and the item stays in Drafts.
• The reply's quoted thread/signature contains inline images referenced by Content-ID (cid:) (e.g. signature social icons), and the add-in's OnMessageSend handler rewrites links to a customer-branded click-tracking CNAME and injects a 1×1 data:image/gif tracking pixel via body.setAsync (coercionType: Html).

What the captured HAR shows

• The send is CreateItem with MessageDisposition=SendAndSaveCopy.
• During compose, GetAttachmentThumbnail for an inline cid: signature image fails (HTTP 400, plus a status 0 network error). The failing attachment id matches the data-custom of that cid: <img> in the body exactly — i.e. an inline cid: attachment is failing to resolve on the client.
• A 403 is returned on the tenant policy lookup graph.microsoft.com/v1.0/organization/<tenant>/serviceProvisioningInfo (analogous to the admin.microsoft.com/centrohost 403 in Outlook Add-in w/ OnMessageSend - There was a problem sending this message because one or more attachments failed to upload. Please try again #6780).
• When the submitted body is clean (no inline images, no injected data: pixel, no add-in internet headers — DataUriCount:0, internetHeaders:[]), the same SendAndSaveCopy returns ResponseCode: NoError. This mirrors Outlook Add-in w/ OnMessageSend - There was a problem sending this message because one or more attachments failed to upload. Please try again #6780's observation that the message sends successfully without the add-in's modified/inline content (and from OWA in Chrome / internal-only).

Suspected root cause

On New Outlook for Windows, when an OnMessageSend add-in rewrites the body via body.setAsync on a reply that contains inline cid: images, the native send appears unable to upload/serialize those inline attachments, and surfaces the generic "one or more attachments failed to upload" error even though there are no user-added file attachments. Removing the inline images / add-in body modifications lets the send succeed.

Ask for Microsoft

1. For the diagnostics below, was the send blocked by inline-attachment (cid:) upload/serialization failing, and why did GetAttachmentThumbnail for that inline image return 400?
2. Does body.setAsync from OnMessageSend on a reply with inline cid: images affect inline-attachment upload on the native send path?
3. Is the serviceProvisioningInfo / compliance 403 on the send path contributing, or incidental?

Diagnostics for correlation

Platform:               New Outlook on Windows (WebView2)
Microsoft Outlook ver:  1.2026.609.400
Client (OWA) version:   20260612016.27  (scriptVer 20260619010.12)
WebView2:               149.0.4022.80 (Stable)
Add-in manifest ver:    3.1.1
Session ID:             5e0c9407-41f6-0f47-8049-f468d3b54149
Microsoft Support ID:   7v6kpg
Send action:            CreateItem / SendAndSaveCopy -> ResponseCode NoError (clean body)
Inline attachment fail: GetAttachmentThumbnail -> HTTP 400 + network error (cid: signature image)

Evidence (HAR, console log, full analysis)

• HAR (zipped): https://github.com/dmitriikashin-outreach/Outlook-Add-in-troubleshooting/blob/main/attachments-failed-to-upload-2026-06-26/repro-har.zip
• Console log: https://github.com/dmitriikashin-outreach/Outlook-Add-in-troubleshooting/blob/main/attachments-failed-to-upload-2026-06-26/console-logs.log
• Full log analysis: https://github.com/dmitriikashin-outreach/Outlook-Add-in-troubleshooting/blob/main/attachments-failed-to-upload-2026-06-26/ai-analysis.md