December 8, 2023 at 01:53PM
Microsoft has acknowledged a new issue affecting Outlook for Microsoft 365 users, causing email-sending problems due to nested folders. The problem is related to an older issue concerning mailboxes with more than 500 shared folders. Workarounds include reducing nested folders, keeping mailbox folders collapsed, and avoiding certain online actions in Outlook. Microsoft is investigating the issue and has provided temporary fixes for affected customers.
From the meeting notes, it is clear that Microsoft has acknowledged a new issue affecting Outlook for Microsoft 365 users, causing email-sending problems for those with too many nested folders. This issue arose from a limit being lifted in 2019 which affected mailboxes with more than 500 shared folders, but it seems that cases with the same number of folders in the primary mailbox were not considered. The advisory from Microsoft suggests that affected users may receive an unexpected Non-Delivery Report with the error code 0x80040305, and that their emails may not reach some or all recipients. Microsoft is currently investigating the issue and has provided some workarounds, including reducing the number of subfolders to under 500 and keeping all mailbox folders collapsed instead of expanded. Additionally, users are advised to avoid certain online actions in Outlook and to restart Outlook immediately for best performance if they use any of these actions. This issue is particularly significant as it affects the ability of Microsoft 365 users to send emails, and Microsoft is actively working to provide solutions for the affected customers.