Outlook Sync Issues Folder Filling Up With Errors? Resync or Rebuild the .ost

Classic Outlook keeps logging entries in the Sync Issues folder (and its Conflicts, Local Failures, and Server Failures subfolders), and what you see in Outlook doesn’t match the web mailbox.

Why: Outlook keeps a local cached copy of your mailbox in an .ost file (Cached Exchange Mode). When that cache can’t reconcile with the server — usually because the .ost is corrupted — the differences get logged into the Sync Issues folder instead of syncing cleanly.

Before clearing or rebuilding: preserve local-only items

Compare Outlook with the web mailbox, including mail, calendar, contacts, Drafts, Outbox and Local Failures. An OST can contain edits or items that never reached the server. Export those items first through File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst) and verify the export. Do this before Clear Offline Items or an OST rebuild, not afterward. Do not proceed if you cannot preserve important local-only data.

Fix 1: Resync the affected folder

If only one folder is misbehaving, you don’t need to touch the whole cache. This is Microsoft’s recommended first step.

1. In the folder list, right-click the problem folder and choose Properties.
2. Click Clear Offline Items, then OK. This empties the local cached copy of that one folder.
3. On the ribbon, open the Send / Receive tab and click Update Folder.

Outlook re-downloads that folder fresh from the server, which clears most one-folder sync errors.

Fix 2: Rebuild the .ost cache

If the errors span many folders, rebuild the whole offline cache so Outlook re-downloads everything.

1. Close Outlook. Open File Explorer, and on the View tab tick File name extensions and Hidden items.
2. Go to C:\Users\<you>\AppData\Local\Microsoft\Outlook.
3. Rename the .ost file (for example Outlook.ost → Outlook.old). If you get a “file is in use” error, end Outlook.exe and Winword.exe in Task Manager first.
4. Restart Outlook. You’ll see Preparing for first use while it re-downloads the mailbox from Exchange. Wait for it to finish.

The rebuilt cache restores server data only. Keep the renamed OST while verifying the result, but use the PST export to preserve or restore local-only items; a renamed OST alone is not a usable export.

Fix 3: Restore preserved local-only data if needed

After the resync or rebuild finishes, compare with the web mailbox and the PST you exported before the repair. Restore any missing exported items through File → Open & Export → Import/Export, choosing Do not import duplicates when importing. Check the recovered folders before removing backups.

FAQ

Is it safe to just delete the Sync Issues messages? Yes — they’re local log entries, not real mail. Deleting them won’t affect your mailbox; if the underlying problem remains they’ll come back, which is your cue to run Fix 1 or 2.

Scanost.exe isn’t on my PC. Correct — the OST Integrity Check Tool shipped only through Outlook 2007. On Outlook 2010 and later, resyncing the folder or rebuilding the .ost (Fixes 1–2) is the supported path.

Sources: Microsoft Learn — Synchronization problems in Outlook and OWA