Microsoft Graph Returning Intermittent 401 Errors for Sending Emails with Large Attachments
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Update - Update from Microsoft:
The cause of the issue has been identified, and a fix has been rolled out. It is expected to be fully deployed by early next week. We apologize for the disruption this has caused.
We will continue to monitor the situation and provide further updates as needed. Thank you for your patience.
Apr 18, 2025 - 12:11 PDT
The cause of the issue has been identified, and a fix has been rolled out. It is expected to be fully deployed by early next week. We apologize for the disruption this has caused.
We will continue to monitor the situation and provide further updates as needed. Thank you for your patience.
Apr 18, 2025 - 12:11 PDT
Update - To mitigate Microsoft's bug that only impacts attachments sent using multipart/form-data, we recommend using application/json instead for attachments under 3MB. This workaround will continue to work effectively even after Microsoft resolves their bug.
For more details, please refer to Nylas’ official guide:
Sending attachments < 3MB via JSON (Nylas API v3)
Apr 18, 2025 - 10:34 PDT
For more details, please refer to Nylas’ official guide:
Sending attachments < 3MB via JSON (Nylas API v3)
Apr 18, 2025 - 10:34 PDT
Update - We have escalated the bug ticket to the highest priority with Microsoft Office 365. However, at the time of this update, they have only acknowledged receipt of the issue. We are actively working to get a response and status update from Microsoft, but they have yet to make a public acknowledgment.
The number of users and organizations reporting these errors—particularly when sending larger attachments via the Graph API—on public Microsoft forums continues to rise. For reference, here is one of the public threads:
https://learn.microsoft.com/en-us/answers/questions/2258885/graph-api-createuploadsession-returns-401-on-uploa
At Nylas, we are exploring potential solutions to help mitigate these errors for our customers while Microsoft works on a fix. We will update this page as soon as we have progress to share.
Apr 18, 2025 - 09:06 PDT
The number of users and organizations reporting these errors—particularly when sending larger attachments via the Graph API—on public Microsoft forums continues to rise. For reference, here is one of the public threads:
https://learn.microsoft.com/en-us/answers/questions/2258885/graph-api-createuploadsession-returns-401-on-uploa
At Nylas, we are exploring potential solutions to help mitigate these errors for our customers while Microsoft works on a fix. We will update this page as soon as we have progress to share.
Apr 18, 2025 - 09:06 PDT
Update - We are still waiting on a response from Microsoft to our support ticket, but continue to find other MS Graph users facing the same errors from MS starting today:
https://learn.microsoft.com/en-us/answers/questions/2258994/issue-with-sending-emails-with-large-attachments-v
Apr 17, 2025 - 12:25 PDT
https://learn.microsoft.com/en-us/answers/questions/2258994/issue-with-sending-emails-with-large-attachments-v
Apr 17, 2025 - 12:25 PDT
Identified - We continue to investigate the root cause, however we are starting to see reports around the Internet of other Microsoft Graph users experiencing this net-new behavior and errors from MS. Below is one example:
https://learn.microsoft.com/en-us/answers/questions/2258885/graph-api-createuploadsession-returns-401-on-uploa
We are also opening a Bug Ticket with Microsoft.
Apr 17, 2025 - 09:46 PDT
https://learn.microsoft.com/en-us/answers/questions/2258885/graph-api-createuploadsession-returns-401-on-uploa
We are also opening a Bug Ticket with Microsoft.
Apr 17, 2025 - 09:46 PDT
Update - We have narrowed the impact of this incident to include only calls to Send for Microsoft Graph for emails with large attachments. We are continuing to investigate why Microsoft started returning these provider errors at approximately 1:00 AM today.
Apr 17, 2025 - 05:37 PDT
Apr 17, 2025 - 05:37 PDT
Update - We are still investigating the cause of this incident.
Apr 17, 2025 - 04:15 PDT
Apr 17, 2025 - 04:15 PDT
Investigating - We are investigating higher levels of 401 errors for email sending. This appears to be limited to Microsoft grants sending emails with attachments.
Apr 17, 2025 - 03:55 PDT
Apr 17, 2025 - 03:55 PDT
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