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.

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.

Because the .ost is just a cache, deleting it is safe — the master copy lives on the server.

Fix 3: Export local-only data before rebuilding

Do this only if you have items that live in Outlook and not on the server (rare with Exchange, but possible).

1. File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst) and save the data.
2. Rebuild the .ost as in Fix 2 and let the server data download.
3. Re-import the .pst, choosing Do not import duplicates so nothing is doubled up.

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