Fix Outlook Desktop Sync Issues Step by Step

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.

Before you start

What you will learn

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.

Requirements

Local administrator access on your Windows PC and email account credentials.
Windows 10 or 11 with classic Outlook Desktop installed.
Local administrator

Good to know

Simple fixes take 5-10 minutes. Profile recreation or large OST rebuilds can take 30-60 minutes.

Quick answer

Most 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.

Send / Receive tab > Work Offline button

Step-by-step tutorial

8 steps

Check Work Offline status and force manual sync

Rule out accidental offline mode.

Outlook > Send / Receive tab > Work Offline

In 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.'

Offline mode often triggers during brief network interruptions. If it keeps re-enabling, check network stability.

Verify account settings and test connection

Confirm server addresses, ports, and authentication.

File > Account Settings > Change

Go 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.

If authentication fails, check whether legacy auth is disabled. Generate an app password if MFA is enforced.

Clear 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.

Cmd

%localappdata%\Microsoft\Outlook

Expected result — Outlook reopens and rebuilds the cache. Folders gradually populate.

Multiple email accounts create multiple OST files. Delete only the one for the affected account if identifiable.

Create a new Outlook profile

Replace a corrupted Outlook profile.

Control Panel > Mail > Show Profiles > Add

Close 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.

Cmd

control mlcfg32.cpl

Expected result — Outlook opens with the new profile and begins syncing.

Export local data first via File > Open & Export > Import/Export. Keep the old profile for several days before deleting.

Repair 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.

Cmd

C:\Program Files\Microsoft Office\root\Office16\scanpst.exe

Expected result — Scanpst.exe reports 'Repair complete.' Recovered items appear in a recovery folder.

Ensure free disk space of at least double the PST file size. Scanpst.exe also works on OST files.

Configure antivirus and firewall exceptions

Ensure security software isn't blocking Outlook connections.

Windows Security > Firewall > Allow an app through firewall

Temporarily 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.

Enterprise antivirus products sometimes have email-scanning modules that interfere with Outlook.

Force folder resync and review cached mode settings

Fix specific stale folders.

File > Account Settings > Change > More Settings > Advanced

Right-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.

If Mail to keep offline is set to a short period, older emails won't appear locally.

Move PST files out of cloud-synced folders

Eliminate corruption from OneDrive file locking conflicts.

File > Account Settings > Data Files

Close 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.

For cloud backup, use a dedicated backup solution that copies the file while Outlook is closed.

How to Confirm Outlook Sync Is Working

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.

Status bar shows 'All folders are up to date.' Test emails arrive within 1-2 minutes.
If status bar shows errors or 'Disconnected,' check network connection and re-verify account settings.
Outlook is fully connected.
OST rebuild in progress.
Network or auth issue.

Troubleshooting

Outlook keeps reverting to offline mode

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

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

Cause: Large mailbox or slow internet.

Leave Outlook running overnight. Temporarily reduce Mail to keep offline to 3 months.

Scanpst.exe repair fails repeatedly

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

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.

How to Fix Microsoft Outlook Desktop Sync Issues