For some time, Microsoft 365 users have encountered a strange issue when trying to send emails via Outlook. Specifically, those with more than 500 folders in their inbox are seeing error code 0x80040305, which indicates a “non-delivery report” – meaning the email failed to reach its recipient.
Microsoft first confirmed the issue in December, and a recent update on September 30th suggests that developers have found a solution. The fix is expected to roll out in upcoming updates, starting with Microsoft 365 Insiders in early October. A final patch for all users is set for release on October 29th, with enterprise users possibly waiting until December 10th.
In the meantime, users affected by the bug can reduce the number of folders in their inbox to below 450 or keep folders collapsed to avoid triggering the error.
