You open Outlook. Your inbox shows 47 unread messages. You click one — it vanishes. You refresh. It’s back. Then gone again. You check Outlook Web App: all messages are there. Your desktop client is stuck in limbo. You’re not offline. You’re not out of space. Something’s misaligned — and it’s silently breaking your workflow.
Most Common Cause
It’s almost always a corrupted Offline Outlook Data (.ost) file — especially after Windows updates, unexpected shutdowns, or large mailbox migrations. The file doesn’t delete itself. It just lies quietly, pretending to sync while serving stale or partial data. I’ve seen this trip up even experienced users who swear their connection is fine. The quick fix? Delete the .ost file and let Outlook rebuild it from scratch. Yes — really. Outlook won’t lose anything. It pulls everything fresh from Exchange or Microsoft 365 servers. Just make sure you’re connected before you start.
Diagnostic Steps
Don’t guess. Diagnose first. Open Outlook and go to
File > Account Settings > Account Settings…. Double-click your email account. In the dialog, click
Change…, then scroll down and uncheck
Use Cached Exchange Mode. Click
Next, then
Finish. Restart Outlook. If sync behaves normally now, cached mode is the culprit — and your .ost file is likely damaged. If problems persist, keep cached mode enabled and move to deeper diagnostics.
Check the status bar at the bottom-right corner of Outlook. Hover over the sync icon (two circular arrows). If it says “Processing” for more than 90 seconds, or shows “Disconnected” while other apps have internet, that’s your first clue. Also press
Ctrl+Shift+Alt+P to open the Outlook Connection Status window — it shows real-time sync activity per folder.
On Mac: Go to
Outlook > Preferences > Accounts, select your account, click
Advanced, and look for red warning icons next to folders. These indicate sync failures — often tied to folder permissions or iCloud interference (yes, iCloud Drive syncing can hijack Outlook’s local cache on macOS).
Fix #1: Rebuild the .ost File (Windows Only)
This works for Outlook 2016, 2019, and Microsoft 365 on Windows. Not available on Mac — Outlook for Mac uses a different storage architecture (.olk15 files, not .ost).
Close Outlook completely. Press
Win+R, type
control.exe /name Microsoft.Mail, hit Enter, then click
Email Accounts. Under
Account Settings, click the
Data Files tab. Select your primary account’s .ost file (usually named something like
yourname@company.com.ost) and click
Remove. Don’t panic — this only deletes the local copy. Your mail stays safe on the server.
Now reopen Outlook. It will prompt you to set up your account again — but skip the wizard. Instead, go to
File > Account Settings > Account Settings…, double-click your account, re-enable
Use Cached Exchange Mode, and click
Next. Outlook begins rebuilding the .ost. Expect 10–45 minutes depending on mailbox size. Let it run uninterrupted — don’t minimize the window.
Surprising tip: If Outlook stalls at “Downloading folder hierarchy”, go to
File > Options > Advanced > Outlook Data File Settings and uncheck
Download shared folders. Shared mailboxes often trigger sync hangs — disable them temporarily, rebuild, then re-enable selectively.
Fix #2: Reset Folder Sync State
This fixes phantom duplicates, missing subfolders, or folders stuck at “Syncing…” forever. It’s faster than rebuilding the whole .ost — and works on both Windows and Mac.
In Outlook desktop (Windows), right-click any folder showing sync issues (e.g., “Sent Items”), choose
Properties, go to the
Synchronization tab, and click
Reset Folder Sync State. Confirm when prompted. Outlook drops its local tracking for that folder and redownloads headers and contents.
On Mac: Right-click the problematic folder >
Re-sync Folder. Watch the progress bar — if it freezes at 99%, force-quit Outlook, hold
Option while reopening, and select
Reset Sync State for Selected Folder from the startup menu.
This also resolves cases where you see two identical folders with slightly different names (“Inbox (1)” vs “Inbox”) — usually caused by Outlook creating fallback copies during failed sync attempts.
Fix #3: Disable Add-ins That Break Sync
Third-party add-ins — especially older CRM plugins, email tracking tools (like Mailtrack or Yesware), and legacy antivirus integrations — inject code directly into Outlook’s sync pipeline. They don’t crash Outlook outright. They just delay or drop sync requests silently.
Go to
File > Options > Add-ins. At the bottom, change the dropdown to
COM Add-ins and click
Go…. Uncheck every add-in except Microsoft’s own (e.g., “Microsoft Teams Meeting Add-in”). Click
OK, restart Outlook, and test sync. If it works, re-enable add-ins one-by-one, testing after each. Note which one breaks it.
On Outlook for Mac, go to
Outlook > Preferences > Add-ins. Disable all third-party items — especially those marked “Not signed” or “Legacy”. Some add-ins (like certain Adobe Sign extensions) only load on Windows, but others (e.g., Grammarly for Outlook) ship cross-platform and cause timeouts on Mac due to slower JavaScript engine handling.
Counterintuitive tip: Even if an add-in shows “Enabled” and “Loaded”, it may be blocking sync in the background. Always test with *all* disabled first — don’t assume “it worked yesterday” means it’s innocent today.
Still Not Working?
If you’ve tried all three fixes and still see sync errors:
• Check your mailbox size. Outlook 365 has a 100 GB soft limit. If you’re at 98.7 GB, sync slows dramatically — especially on older hardware. Archive old items manually via
File > Info > Tools > Clean Up Old Items.
• Verify your Exchange Online version. Outlook 2016 clients connecting to newer Exchange Online tenants sometimes need a registry tweak (Windows only): set
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Cached Mode\DisableMAPIOverHTTP to
0 — but only do this if your IT team confirms MAPI over HTTP is enabled on your tenant.
• Rule out group policy interference. Corporate environments sometimes enforce sync throttling via GPO. Run
gpresult /h gpreport.html from Command Prompt and search for “Outlook Sync Delay” or “Cached Exchange Mode Policy”.
At this point, contact your IT team — but send them this info first:
• Exact error message (e.g., “Error 0x8004010F”, “Cannot synchronize folder”)
• Whether it affects one folder or all folders
• Whether the issue occurs in Outlook Web App (it shouldn’t — if it does, it’s server-side)
• Screenshots of the Connection Status window (
Ctrl+Shift+Alt+P)
| Problem State |
What Happens |
Result After Fix |
| Corrupted .ost file |
New emails appear then vanish; Sent Items don’t reflect recent sends |
All folders show current state; no more ghost messages |
| Stuck folder sync |
“Drafts” shows 0 items despite having 12 drafts; sync icon spins indefinitely |
Drafts count updates instantly; folder loads in under 2 seconds |
| Add-in conflict |
Sync completes for Inbox but fails on Calendar and Contacts; no error shown |
Calendar and Contacts sync within 30 seconds of opening |
| Shared mailbox timeout |
Main mailbox syncs fine; shared mailbox “Marketing Team” shows last synced 3 days ago |
Shared mailbox syncs fully within 1 minute; shows “Last synced: Just now” |
| iCloud Drive interference (Mac) |
Outlook crashes on launch; Console logs show “file lock conflict on Outlook.sqlite” |
Outlook launches reliably; sync runs without interruption |
