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The error occurs only when sending to multiple recipients and the non-delivery report shows a generic client operation failure. From the available information, the most likely cause is a service-side issue in the Outlook/Exchange infrastructure rather than a local Outlook profile problem.
Because the issue appears only in specific scenarios (multiple recipients) and Outlook works for other accounts on the same PC, the recommended approach is:
- Use the Outlook web interface (Outlook.com or Outlook on the web) with the same account and try sending the same message to the same set of recipients. If it works there, the problem is isolated to the desktop client.
- If the problem persists in both desktop and web, it is almost certainly a backend service issue. In that case, wait and try again later, as similar issues have been reported where Microsoft’s mail service temporarily fails to deliver messages from certain senders or to certain providers.
- If the problem continues, run the Classic Outlook troubleshooters to rule out local profile or configuration issues:
- Go to the Classic Outlook troubleshooters page and run the appropriate troubleshooter for sending/receiving issues.
- If none of the above resolves the problem, contact Microsoft support so they can check the mailbox and service-side logs for the specific error code and message trace.
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