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Is Office 365 Version 2505 Destroying Your Emails? Critical Bug Causes Devastating Message Loss
A critical bug has emerged in Microsoft Office 365 version 2505 that causes emails to vanish when users move messages from shared mailboxes to subfolders while operating in cached mode. This issue was first reported by German users on June 3, 2025, and has since been confirmed by multiple users experiencing identical problems with Outlook classic.
The Email Disappearing Problem
The core issue manifests when users attempt to move emails from the inbox of a shared mailbox to any subfolder – the messages simply fail to reach their intended destination. This behavior was initially discovered when a colleague contacted a user named David on June 2, 2025, alerting him to missing emails.
What makes this particularly concerning is the rapid spread of the problem. Initially, David’s colleagues were skeptical of his reports, but their attitudes changed dramatically when two colleagues updated to Office 365 version 2505 on June 3, 2025, and immediately began experiencing identical email loss issues.
Technical Investigation and Testing
Extensive troubleshooting efforts have been conducted to isolate this bug:
- Cache dependency confirmed: The problem only occurs when cached mode is active
- Browser testing: Moving emails directly through web browsers works normally
- OST file recreation: Complete recreation of OST files at C:\Users\*Username*\AppData\Local\Microsoft\Outlook failed to resolve the issue
- Full folder reconstruction: Even recreating entire Outlook folders did not eliminate the problem
Additional symptoms reported include folder structures in shared mailboxes appearing to “flicker” or collapse unexpectedly.
Current Workaround Solution
The only effective temporary solution identified involves disabling cached mode for shared mailboxes by removing the checkmark from the option that includes shared mailboxes in the offline cache. While this resolves the email loss issue, it may impact performance for users who rely heavily on offline access to shared mailbox content.
Office 365 Issues
This email loss bug occurs against a backdrop of ongoing Office 365 stability concerns. Microsoft’s release notes for Current Channel show a pattern of resolved issues across recent versions, including problems with Excel performance, Outlook encryption failures, and various security-related fixes. The company has been actively addressing bugs through regular updates, with Version 2505 released on June 3, 2025, containing “various bug and performance fixes”.
Impact and Recommendations
This bug represents a significant risk for organizations relying on shared mailboxes for collaborative email management. The complete loss of moved emails could result in:
- Critical communication gaps
- Compliance and audit trail issues
- Potential data loss incidents
- Workflow disruptions
Organizations using Office 365 version 2505 should immediately implement the workaround by disabling cached mode for shared mailboxes until Microsoft releases an official fix. IT administrators should also consider postponing updates to version 2505 for users who heavily depend on shared mailbox functionality.