The Sync Issues folder in Outlook contains logs of synchronization errors between Outlook and the mail server (Exchange or Microsoft 365). It has three subfolders, each representing a different type of sync failure.
Part 1. Understanding the Three Subfolders
| Subfolder | Contains | Urgency |
|---|---|---|
| Sync Issues | General sync error logs | Low — informational |
| Server Failures | Items the server couldn't process | Medium — check if emails missing |
| Local Failures | Items Outlook couldn't send to server | High — emails may not be sent |
⚠️ Important: Check the Local Failures subfolder — items here failed to sync to the server, meaning emails or calendar items may exist only locally and not on the server. If you switch devices or reinstall Outlook, these items could be lost.
Part 2. Common Causes of Sync Issues
| Cause | Symptom | Fix |
|---|---|---|
| Corrupted OST file | Consistent sync errors | Delete and rebuild OST |
| Network interruptions | Intermittent errors | Check internet connection stability |
| Large attachments | Server rejection errors | Reduce attachment size |
| Mailbox over quota | Items can't sync | Archive or delete emails |
| Exchange/M365 server issues | All users affected | Wait for server recovery |
| Outdated Outlook version | Known sync bugs | Update Outlook |
Part 3. Fix by Rebuilding the OST File
The most effective fix for persistent sync errors:
- Close Outlook completely
- Open Control Panel → Mail → Email Accounts → Data Files
- Note the OST file path
- Close the dialog
- Navigate to the OST path in File Explorer
- Rename the
.ostfile to.ost.bak(renames, doesn't delete) - Reopen Outlook — it creates a fresh OST and re-downloads from server
💡 Tip: Renaming rather than deleting the OST preserves it as a backup. If the fresh OST causes unexpected issues, rename the .bak back to .ost.
💡 Tip: Before rebuilding the OST, check the size of the Sync Issues folder — if it's only a few dozen items, the errors may be intermittent and harmless. Only rebuild the OST if sync errors are persistent, growing rapidly, or if you notice actual email delivery problems alongside the sync logs.
Part 4. Delete Sync Issue Logs Safely
Sync issue logs are informational — deleting them doesn't affect emails:
- In Outlook's folder list: Sync Issues → select all items → Delete
- Empty the Deleted Items folder to free space
- Repeat for Server Failures and Local Failures subfolders (check Local Failures for any items you might need first)
💡 Tip: To select all items in the Sync Issues folder quickly, click the first item then press Ctrl+A to select all, then Delete. For very large Sync Issues folders (tens of thousands of items), this is much faster than deleting in batches.
🗣️ r/sysadmin guidance: "Sync Issues logs are noise — they're generated automatically and don't need action in most cases. Delete them freely. The exception is Local Failures — those might contain emails that haven't reached the server yet."
🗣️ r/techsupport user: "Sync Issues folder had 50,000 items and was consuming 2 GB. All were harmless error logs. Deleted everything, rebuilt the OST, and sync errors stopped. Fresh OST resolved the underlying corruption."
Part 5. When Sync Issues Indicate Deeper Problems
Persistent sync errors that survive OST rebuild may indicate:
- Exchange server configuration issues (admin-level fix)
- Microsoft 365 account access problems (check admin portal)
- Local Windows profile corruption (create new Windows user profile)
For any emails you're concerned about losing due to sync failures, Ritridata can recover .pst and .ost files if they were accidentally deleted.
FAQ
Is it safe to delete items in the Sync Issues folder? Yes — sync issue logs are automatically generated informational messages. Deleting them doesn't affect emails, contacts, or calendar items. Exception: check Local Failures for items that may not have reached the server.
Why does my Sync Issues folder keep filling up? Recurring sync errors usually indicate a corrupted OST file, network instability, or a persistent server-side issue. Fix the root cause (usually OST rebuild) to stop new entries from being generated.
Do sync issues mean I'm losing emails? Usually not — most sync errors are temporary and resolve themselves. Check the Local Failures subfolder specifically: items there failed to upload to the server and may not be on other devices.
How do I stop Sync Issues from being shown in Outlook's folder list? Outlook → View → Folder Pane → check "Show Sync Issues Folder" setting. Hiding it doesn't delete the logs — it just removes it from the navigation pane.