For several months, Microsoft 365 users with extensive Outlook inboxes have encountered an unusual problem when sending emails. The error, marked by code 0x80040305, affects users with over 500 folders in their inbox and results in emails failing to deliver to recipients.
This issue was acknowledged by Microsoft in December, but as of September 30th, a fix is on the horizon. Microsoft 365 Insiders will be the first to receive the update in early October, followed by a general patch on October 29th. Enterprise customers may have to wait until December 10th for the full resolution.
To mitigate the issue before the update, Microsoft advises reducing inbox folders to under 450 or keeping them collapsed.