Fix Outlook and Office 365 Sync Issues (2025 Complete Guide)

When Outlook stops syncing your emails or calendar with Office 365, your workflow freezes. Whether you’re missing messages, seeing “synchronizing subscribed folders” errors, or Outlook stuck in offline mode — this 2025 guide shows every way to fix Outlook and Office 365 sync issues safely and permanently.

Why Outlook Fails to Sync With Office 365

Several common causes trigger sync failures:

• Corrupt Outlook data (OST/PST) files
• Authentication or password token problems after updates
• IMAP folder mismatches or server timeouts
• Disabled Cached Exchange Mode
• Faulty add-ins and antivirus interference

Step 1 – Verify Your Internet and Server Status

1. Open a browser and sign in to Outlook Web Access. If mail loads there, your Office 365 account is fine.
2. Toggle Work Offline in Outlook (Send/Receive → Work Offline) to refresh the connection.
3. Restart your router and disable VPN or proxy temporarily.

Step 2 – Update Outlook and Office 365

Outdated builds often cause sync loops. Go to File → Office Account → Update Options → Update Now. Reopen Outlook after updates install.

Step 3 – Enable Cached Exchange Mode

Cached mode lets Outlook store local copies of your mail for smooth sync even on slow networks:

1. File → Account Settings → Account Settings.
2. Select your Exchange account → Change.
3. Check Use Cached Exchange Mode → Next → Finish → Restart Outlook.

Step 4 – Repair Outlook Data Files

Corrupted OST or PST files block synchronization. Use the built-in tool SCANPST.EXE to repair them or follow our detailed guide below:

Repair Outlook Data File (.OST/.PST) to Fix Sync Errors

Step 5 – Reset IMAP Folder Path (For IMAP Accounts)

1. File → Account Settings → Change → More Settings → Advanced tab.
2. Set Root Folder Path to INBOX.
3. Save changes and restart Outlook.

This aligns folder hierarchy with Gmail, Yahoo, or custom IMAP servers and resolves most stuck sync states. For a full walk-through, see Fix Outlook Stuck on Synchronizing Subscribed Folders.

Step 6 – Check Send/Receive Groups and Sync Settings

1. Send/Receive → Send/Receive Groups → Define Groups.
2. Edit your account → Ensure “Included in this group” is checked.
3. Reduce the sync interval to every 10 minutes.

Step 7 – Disable Faulty Add-ins

1. File → Options → Add-ins → COM Add-ins → Go.
2. Uncheck all third-party add-ins and restart Outlook.
3. If sync resumes, enable add-ins one by one to find the offender.

Step 8 – Recreate Your Outlook Profile

1. Control Panel → Mail → Show Profiles.
2. Click Add → Create a new profile → Enter credentials.
3. Set “Always use this profile” and remove the old one if needed.

Advanced Fixes for Office 365 Enterprise Users

1. Clear Autodiscover Cache

del %localappdata%\Microsoft\Outlook\Autodiscover*

2. Repair Office 365 Installation

1. Control Panel → Programs and Features.
2. Select Microsoft Office → Change → Quick Repair or Online Repair.

3. Use Microsoft Support and Recovery Assistant (SaRA)

Download the tool from Microsoft and run the Outlook diagnostic to auto-detect server and account issues.

Step 9 – Verify Mailbox Size and Quota

Large mailboxes slow sync dramatically. Check folder sizes under File → Info → Mailbox Settings. Archive old emails and empty Deleted Items regularly.

Step 10 – Rebuild Offline Cache (OST)

1. Close Outlook → Navigate to %localappdata%\Microsoft\Outlook.
2. Rename the .OST file → Restart Outlook → It will rebuild a fresh cache.

Preventive Maintenance Tips

• Always close Outlook gracefully before shutting down your PC.
• Keep Windows and Office 365 fully updated.
• Back up PST and OST files monthly.
• Limit add-ins and avoid third-party sync connectors that overwrite server data.

Related Guides and Resources

• Fix Outlook Stuck on Synchronizing Subscribed Folders
• Repair Outlook Data File (.OST/.PST) to Fix Sync Errors
• Email Setup and Configuration Guide

Conclusion

Outlook and Office 365 sync issues can be frustrating but are almost always fixable. Start with basic network checks, enable Cached Exchange Mode, and repair your data files if needed. For persistent problems, rebuild your Outlook profile or use FixTechGuide’s free diagnostic tools to automate detection of authentication and IMAP errors in seconds.