Unusual Outlook bug that prevents sending emails is now solved

For a while now, some Microsoft 365 users have been running into problems when sending emails via Outlook.

More specifically, users who have more than 500 folders (!) in their Outlook inbox have been seeing a dialog box with error code 0x80040305. This obscure error code points to a “non-delivery report,” meaning the email didn’t reach its intended recipients.

The issue was first confirmed by Microsoft back in early December, but according to a September 30th update, it appears that Microsoft developers now believe they have found a solution — and that means it should be fixed in an upcoming update.

Said update is likely to come in the coming weeks, with Microsoft 365 Insiders getting it first. Those on the Beta Channel will get it some time in early October, with the final patch expected to release on October 29th. Enterprise users, however, may need to wait until December 10th.

If you’re affected by the bug and can’t wait for the fix, you can try to reduce the amount of inbox folders in the meantime. (Microsoft recommends no higher than 450 folders.) Also, if you have hundreds of folders, you should keep them collapsed instead of expanded.