​Microsoft has fixed a known issue affecting Outlook for Microsoft 365 users that caused problems sending emails for those with too many nested folders.

When it first acknowledged the email-sending issues in December, the company linked them to an older problem related to mailboxes with more than 500 shared folders—a limit removed in 2019. However, Redmond now believes the issue also affects users with a similar number of folders in their primary mailbox.

Microsoft says users trying to send emails from the Outlook desktop app may receive an unexpected Non-Delivery Report (NDR) with the 0x80040305 error code.

“This issue is like the prior 500 folder limit that the Outlook Team fixed, Lifting the 500 Folder Limit in Outlook,” the company said. “That solution targeted shared folders but did not include the scenario occurring in the user’s primary mailbox.”

Affected users will also be notified that their emails did not reach “some or all” recipients, with the NDR message advising them to resend it later or contact their network administrator.

The Outlook Team has developed a fix for this problem, which will be rolled out to the following release channels later this month or in December:

• Beta Channel: Version 2410 (Build 18127.15020), expected early October 2024
• Current Channel Preview: Version 2410 (Build 18129.20000), expected October 9, 2024
• Current Channel: Version 2410 (Build 18129.20000), expected October 29, 2024
• Monthly Enterprise Channel: Version 2410 (Build 18129.20000), expected December 10, 2024

Workaround also available

In the meantime, until the affected users’ software is fixed, Microsoft provides a temporary workaround requiring them to reduce the number of folders with subfolders to below 500, aiming for 450 nested folders.

Another temporary solution to these Outlook email-sending problems is to keep all mailbox folders collapsed rather than expanded.

“If you can’t do either of the above, avoid subsequent online actions in Outlook. Such actions include using the buttons for ‘View on Server’ and ‘Click here to view more on Microsoft Exchange,’ or use of ‘Include older results’ when searching,” Microsoft said.

“If you use any of these actions, restart Outlook immediately for best performance.”

Earlier this month, the company also addressed a bug causing Microsoft 365 apps like Outlook, Word, Excel, and OneNote to crash while typing or spell-checking a text.

In August, Microsoft also shared temporary workarounds for known issues triggering Gmail sign-in issues for classic Outlook users and causing Outlook to crash after opening.