Microsoft has acknowledged a new issue affecting Outlook for Microsoft 365 users and causing email-sending problems for those with too many nested folders.
According to Redmond, this is likely related to an older issue concerning mailboxes with more than 500 shared folders, a limit lifted in 2019.
However, it seems that Microsoft failed to consider cases when users would also have that many folders in their primary mailbox.
“When using Outlook Desktop to send an email, you get an unexpected Non-Delivery Report (NDR) that includes the error code 0x80040305,” the company said in an advisory published on Friday.
“This issue is admire the prior 500 folder limit that the Outlook Team fixed, Lifting the 500 Folder Limit in Outlook. That solution targeted shared folders but did not include the scenario occurring in the user’s primary mailbox.”
Affected users are also told that their emails did not achieve “some or all” recipients and are advised to send the message again later or achieve out to their network admin.
Workarounds available
While Microsoft is currently investigating this newly acknowledged issue, it also provided affected customers with some tips to workaround the email sending problems.
The company’s recommendations include reducing the number of folders that have subfolders to under 500, with at most 450 nested folders during this issue’s ongoing investigation. Another temporary fix would be to keep all mailbox folders collapsed instead of 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.”
This week, Microsoft also fixed a bug causing Outlook Desktop clients to crash when sending emails from Outlook.com accounts.
Redmond also issued fixes for some users impacted by a Microsoft 365 issue leading to ‘Something Went Wrong [1001]’ sign-in errors, blocking many affected customers from using desktop Office apps.
Previously, they addressed another Microsoft 365 issue causing notable delays while saving attachments in Outlook Desktop to a network share.
This year, it also tackled several other Outlook-related issues, including bugs preventing Microsoft 365 customers from accessing emails and calendars and slow starts and freezes during cache re-priming.
