How to Fix Microsoft Outlook Desktop Sync Issues
Fix Outlook Desktop sync failures with this step-by-step guide covering offline mode, OST cache repair, profile recreation, antivirus exceptions, and PST file handling for Microsoft 365.
- Difficulty
- Intermediate
- Time required
- 15-45 minutes
- Steps
- 8
- Platform
- Microsoft Outlook
Table of contents
Quick Answer
Go to the stepsMost Outlook sync failures are caused by offline mode being accidentally enabled, a corrupted OST cache file, or a broken Outlook profile. Check the Send/Receive tab for the Work Offline button first. If that's not the issue, delete the OST file at %localappdata%\Microsoft\Outlook to force a rebuild from the server.
- Check and disable Work Offline mode in Send/Receive tab.
- Verify account settings and test connection.
- Delete OST files to force a cache rebuild.
- Create a new Outlook profile if needed.
- Run Scanpst.exe on corrupted PST files.
Send / Receive tab > Work Offline buttonExpected result: Outlook syncs emails, calendar, and contacts within minutes.
Key takeaways
- You'll learn how to diagnose and fix the most common Outlook Desktop sync failures, from toggling offline mode to rebuilding the OST cache and recreating a corrupted profile.
- Outlook sync failures block email, calendar, and contact access, directly impacting productivity. Most fixes take minutes when you know the right order to troubleshoot.
- Check offline mode first, rebuild the OST cache second, and create a new profile as a last resort. Never delete PST files.
Introduction
Outlook Desktop sync issues are one of the most common productivity disruptions in Microsoft 365 environments. The symptoms vary: emails stop appearing, the client gets stuck showing 'Working Offline,' calendar changes don't propagate, or connections fail intermittently with no clear error message.
Most sync failures trace back to a handful of causes. Offline mode gets enabled accidentally, the local OST cache file becomes corrupted after an improper shutdown, the Outlook profile breaks, or antivirus software silently blocks connections to Exchange Online. PST files stored in cloud-synced folders like OneDrive add another common failure mode.
This guide walks through the fixes in order of likelihood and invasiveness. Start with step 1 and work forward. Per Microsoft's official support documentation on repairing Outlook data files, the Inbox Repair Tool (Scanpst.exe) and OST deletion are the two primary repair methods for data file corruption.
Who this is for: Users and IT admins running classic Outlook Desktop (2016, 2019, 2021, or Microsoft 365 Apps) on Windows, connected to Microsoft 365, Exchange, or IMAP accounts.
Before you start
- Access
- Local administrator access on your Windows PC and email account credentials.
- Required roles
- Local administrator
- Local administrator on the Windows PC
- Required licenses
- Microsoft 365 or standalone Outlook license
- Environment
- Windows 10 or 11 with classic Outlook Desktop installed.
- Vendor
- Microsoft
- Administrator permissions required
- Backup required
Simple fixes take 5-10 minutes. Profile recreation or large OST rebuilds can take 30-60 minutes.
Warning: Create a backup of PST files before any repair
PST files contain local data not stored on the server. Verify their location and create a backup before deleting OST files or creating a new profile. Deleting a PST file causes permanent data loss.
Note: OST deletion triggers full re-download
Deleting the OST file forces Outlook to re-download your entire mailbox. For large mailboxes this can take over an hour.
1Check Work Offline status and force manual sync
Rule out accidental offline mode.
Outlook > Send / Receive tab > Work OfflineIn Outlook, click the Send / Receive tab. If the Work Offline button appears highlighted, click it once to disable offline mode. Then click Send/Receive All Folders (or press F9) to trigger an immediate sync.
Check the bottom-right corner of Outlook for a disconnected plug icon or 'Working Offline' text.
Expected result: Work Offline button returns to normal. Status bar shows 'All folders are up to date.'
Note
Offline mode often triggers during brief network interruptions. If it keeps re-enabling, check network stability.
2Verify account settings and test connection
Confirm server addresses, ports, and authentication.
File > Account Settings > ChangeGo to File > Info > Account Settings > Account Settings. Select the account, click Change. For Microsoft 365: incoming server outlook.office365.com port 993 (IMAP/SSL), outgoing smtp.office365.com port 587 (STARTTLS). Click Test Account Settings.
Expected result: All test items show green checkmarks.
Note
If authentication fails, check whether legacy auth is disabled. Generate an app password if MFA is enforced.
3Clear Outlook cache and rebuild OST files
Force Outlook to rebuild its local mailbox cache.
Close Outlook completely. Confirm no OUTLOOK.EXE process in Task Manager. Press Win + R, type %localappdata%\Microsoft\Outlook, press Enter. Delete all .ost and .oab files. Do not delete .pst files.
Restart Outlook. It will re-download your mailbox from the server. This takes 10-30 minutes for average mailboxes.
%localappdata%\Microsoft\OutlookExpected result: Outlook reopens and rebuilds the cache. Folders gradually populate.
Note
Multiple email accounts create multiple OST files. Delete only the one for the affected account if identifiable.
4Create a new Outlook profile
Replace a corrupted Outlook profile.
Control Panel > Mail > Show Profiles > AddClose Outlook. Press Win + R, type control mlcfg32.cpl. Click Show Profiles > Add. Name the new profile, enter your email, and let Outlook auto-configure. Select Always use this profile and choose the new one.
control mlcfg32.cplExpected result: Outlook opens with the new profile and begins syncing.
Note
Export local data first via File > Open & Export > Import/Export. Keep the old profile for several days before deleting.
5Repair PST files using Scanpst.exe
Fix corrupted PST files containing local data.
Locate Scanpst.exe at C:\Program Files\Microsoft Office\root\Office16\scanpst.exe. Close Outlook. Run as administrator. Browse to your PST file, click Start. If errors found, check Make a backup before repairing, then click Repair.
Per Microsoft Support, you may need to run the tool several times until it reports no errors found.
C:\Program Files\Microsoft Office\root\Office16\scanpst.exeExpected result: Scanpst.exe reports 'Repair complete.' Recovered items appear in a recovery folder.
Note
Ensure free disk space of at least double the PST file size. Scanpst.exe also works on OST files.
6Configure antivirus and firewall exceptions
Ensure security software isn't blocking Outlook connections.
Windows Security > Firewall > Allow an app through firewallTemporarily disable antivirus and firewall to test. If Outlook syncs, add exclusions for OUTLOOK.EXE and the Outlook data folder. Allow outbound ports 993 (IMAP SSL), 587 (SMTP STARTTLS), and 443 (HTTPS/OAuth). Re-enable security software immediately.
Expected result: Outlook syncs with security software re-enabled and exceptions configured.
Note
Enterprise antivirus products sometimes have email-scanning modules that interfere with Outlook.
7Force folder resync and review cached mode settings
Fix specific stale folders.
File > Account Settings > Change > More Settings > AdvancedRight-click the stale folder, select Properties > Clear Offline Items > OK. Then right-click again and select Update Folder. Check global settings: File > Account Settings > Change > More Settings > Advanced. Ensure Use Cached Exchange Mode is checked and Mail to keep offline is set to All.
Expected result: Affected folders re-download within 5-10 minutes.
Note
If Mail to keep offline is set to a short period, older emails won't appear locally.
8Move PST files out of cloud-synced folders
Eliminate corruption from OneDrive file locking conflicts.
File > Account Settings > Data FilesClose Outlook. Move .pst files from OneDrive or synced folders to C:\Users\[username]\Documents\Outlook Files. In Outlook: File > Account Settings > Data Files. Remove old cloud-path reference, add the moved local file.
Expected result: Outlook responds immediately without freezing.
Note
For cloud backup, use a dedicated backup solution that copies the file while Outlook is closed.
How to Confirm Outlook Sync Is Working
Verify fix
Send yourself a test email from a phone or webmail. It should appear within 1-2 minutes. Check the status bar for 'All folders are up to date.' Verify calendar sync by creating a test event in Outlook Web App.
If you rebuilt the OST cache or created a new profile, monitor Outlook for the first hour as it re-downloads your mailbox.
Normal result: Status bar shows 'All folders are up to date.' Test emails arrive within 1-2 minutes.
Abnormal result: If status bar shows errors or 'Disconnected,' check network connection and re-verify account settings.
Successful sync
All folders are up to date
Outlook is fully connected.
Partial sync
Some folders still downloading
OST rebuild in progress.
Persistent failure
Disconnected
Network or auth issue.
Troubleshooting
Outlook keeps reverting to offline mode
Warning
Cause: Unstable network or Group Policy.
Check network stability with ping outlook.office365.com -t. Verify no Group Policy forces offline behavior.
Authentication error in test
Warning
Cause: Legacy auth disabled or missing app password.
Generate an app password from mysignins.microsoft.com. Remove and re-add the account for a fresh OAuth flow.
OST rebuild extremely slow
Note
Cause: Large mailbox or slow internet.
Leave Outlook running overnight. Temporarily reduce Mail to keep offline to 3 months.
Scanpst.exe repair fails repeatedly
Warning
Cause: Severe PST corruption.
Run Scanpst.exe up to 5-8 times. If it still fails, try a third-party recovery tool or restore from backup.
Outlook freezes when opening items
Critical
Cause: PST file stored in OneDrive.
Move the PST file to local storage immediately. Update the path in Account Settings > Data Files.
Frequently asked questions
Is it safe to delete OST files?
Yes. OST files are local cache copies. Outlook rebuilds them automatically from the server. Never delete PST files.
How long does OST rebuild take?
Typically 10-30 minutes for average mailboxes. Large mailboxes with years of data can take over an hour.
What is the difference between OST and PST?
OST files are cached server data, safe to delete. PST files contain local-only data. Deleting PST files causes permanent data loss. Use Scanpst.exe to repair corrupted PST files.
Should I store PST files in OneDrive?
No. Microsoft warns against it. Both Outlook and OneDrive compete for file access, causing corruption and freezing. Keep PST files on local storage.
Why does Outlook keep going offline?
Common causes: unstable network, VPN disconnects, Group Policy settings. Check network stability with a continuous ping test.
Where is Scanpst.exe?
For Outlook 2016/2019/2021/M365: C:\Program Files\Microsoft Office\root\Office16\scanpst.exe or the x86 equivalent.
Conclusion
Most Outlook sync issues resolve with disabling offline mode, rebuilding the OST cache, or creating a new profile. For PST corruption, use Scanpst.exe. Antivirus exclusions and moving PST files out of cloud-synced folders address remaining causes.
Check offline mode first, rebuild the OST cache second, and create a new profile as a last resort. Never delete PST files.
Send / Receive tab > Work Offline (toggle off) > Send/Receive All FoldersSources3
