Microsoft Graph Returning Intermittent 401 Errors for Sending Emails with Large Attachments
Incident
Report for Nylas v3
Resolved
Update on Microsoft's Graph API Fix
The issue impacting the sending of emails with large attachments via the Graph API using multipart/form-data due to a Microsoft bug has been addressed. It appears the fix is now fully rolled out, and there have been no errors of this specific type in over 12 hours in both the US and EU regions.
We are confident that the issue has been resolved, but we will continue to monitor the system to ensure stability.
Thank you for your patience throughout this incident.
The issue impacting the sending of emails with large attachments via the Graph API using multipart/form-data due to a Microsoft bug has been addressed. It appears the fix is now fully rolled out, and there have been no errors of this specific type in over 12 hours in both the US and EU regions.
We are confident that the issue has been resolved, but we will continue to monitor the system to ensure stability.
Thank you for your patience throughout this incident.
Monitoring
Update on Microsoft's Graph API Fix
We are seeing significant progress with the fix rolled out by Microsoft for their issue impacting the sending of emails with large attachments via the Graph API using multipart/form-data. Over the past 48 hours, the error rate has dropped substantially, and we are monitoring the improvements closely.
US Region: The error rate has decreased to 0.047% over the past 24 hours.
EU Region: The error rate has decreased to 0.060% over the same period.
We will continue to monitor the situation and provide updates as needed. Thank you for your patience as Microsoft works to fully deploy the fix.
We are seeing significant progress with the fix rolled out by Microsoft for their issue impacting the sending of emails with large attachments via the Graph API using multipart/form-data. Over the past 48 hours, the error rate has dropped substantially, and we are monitoring the improvements closely.
US Region: The error rate has decreased to 0.047% over the past 24 hours.
EU Region: The error rate has decreased to 0.060% over the same period.
We will continue to monitor the situation and provide updates as needed. Thank you for your patience as Microsoft works to fully deploy the fix.
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.
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.
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)
For more details, please refer to Nylas’ official guide:
Sending attachments < 3MB via JSON (Nylas API v3)
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.
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.
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
https://learn.microsoft.com/en-us/answers/questions/2258994/issue-with-sending-emails-with-large-attachments-v
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.
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.
Update
We have narrowed the impact of this incident to include only calls to Send for Microsoft Graph for emails with large attachments using multipart/form-data (calls using application/json appear to be unaffected). We are continuing to investigate why Microsoft started returning these provider errors at approximately 1:00 AM today, and why the error messages themselves do not contain any error information.
Update
We are still investigating the cause of this incident.
Investigating
We are investigating higher levels of 401 errors for email sending. This appears to be limited to Microsoft grants sending emails with attachments.
This incident affected: Nylas Application Services (API v3).