How to Fix Outlook Synchronization Log Issues?

Outlook, when set-up using an Exchange Server or Outlook.com account, automatically synchronizes the emails, contacts, calendar, and other items in the local cache (.OST) with the mailbox on the server and vice-versa. This way the information remains up-to-date in Outlook and on the mailbox server. But sometimes, users encounter discrepancies between their Outlook client and the mailbox server, such as outdated or missing messages and mail items. This happens due to synchronization issues or network failure. Due to this, users are not able to send or receive new emails, experience delayed email delivery, or error messages while synchronizing the mail items manually.

Outlook synchronization issues and errors occur when Outlook cannot establish a secure connection with the mailbox server or encounters issues while synchronizing the mail items with the mailbox server. There are various reasons that can lead to such issues. Let’s understand these reasons and see how to fix the Outlook synchronization log issues.

Reasons for Outlook Synchronization Log Issues

Synchronization issues in Outlook can occur due to the following reasons:

• Network or connectivity issues
• Lack of permissions
• Large or oversized mailbox or OST file
• Inaccessible or damaged OST file
• Corrupt Outlook profile
• Conflict with security apps, such as firewall or antivirus
• Incompatible or faulty add-ins

Solutions to Fix Outlook Synchronization Log Issues

Whenever Outlook fails to synchronize the mail items or folder stored in the OST file with the mailbox server, it generates a synchronization log stored in the Sync Issues folder. This hidden folder contains three other synchronization issues subfolders: Conflicts, Local Failures, and Server Failures. You can check the Sync Issues folder and its subfolders for any messages and use the information provided in the message to troubleshoot the synchronization issues. If the issue is not resolved, then follow the below solutions to resolve the Outlook synchronization log issues.

1. Turn Off Cached Exchange Mode

Turning off Cached Exchange mode will disable the Sync Issues folder and help resolve Synchronization log issues. However, you need a reliable internet connection to work in online mode. The steps to disable Cached Exchange Mode are as follows:

• In Outlook, navigate to File > Account Settings > Account Settings.
• Select your Exchange or Microsoft 365 profile and click Change.
• Uncheck the Use Cached Exchange Mode checkbox.
• Close the window and restart Outlook.

This will disable the Cached Exchange Mode and Outlook will now connect directly with the mailbox server.

2. Reduce Mailbox Size

If the OST file gets large, it develops inconsistencies and errors that can trigger sync issues. By reducing the mailbox size, you can make space for new mail items, optimize Outlook performance, and resolve synchronization issues. To reduce the mailbox size, you can archive old mail items from your mailbox or oversized OST file to a local PST. The steps are as follows:

• Open Outlook and go to the File tab.
• Select Info > Cleanup Tools > Archive.
• Select Archive this folder and all subfolder options
• Choose the mailbox items/folders you want to archive.

• Select a date in the Archive items older than box.
• Click Browse to choose a save location.
• Make sure to mark the “Do not auto-archive” checkbox. Then, click OK.

You can also empty the Deleted Items or Trash folder and delete all the messages from the Spam or Junk email folder to reduce the mailbox/OST file size.

3. Update Outlook

Updating Outlook resolves any bugs or incompatibility issues that might be responsible for the synchronization errors and issues. You can check and update Outlook by following these steps:

• Open Outlook and go to File > Office Account > Update Options.
• From the available options, choose Update Now.

• Outlook will launch a wizard and download the recent updates if available. Finish the update wizard to complete the process.

After the update, restart Outlook and see if the issue is resolved.

4. Remove Faulty Add-ins

Any faulty or incompatible add-ins can cause various issues in Outlook. Start Outlook in safe mode to check if the error is caused by some incompatible or faulty third-party add-ins. The steps are as follows:

• Close Outlook if running.
• Press Windows + R, type Outlook.exe /safe, and press the Enter key.

• Choose your profile if prompted. This will start Outlook in safe mode.
• Go to File > Options > Add-Ins.
• Click the Go button beside the Manage COM Add-Ins option.
• Uncheck all add-ins to disable them and click OK.

• Close Outlook and then start it normally.

Now, if you don’t see synchronization log issues, it means one or more add-ins were responsible for the problem.

To find the problematic add-in,

• Enable each add-in and observe Outlook.
• As soon as it starts creating synchronization logs or errors, uninstall the last activated add-in.

5. Disable/Adjust Antivirus or Firewall Settings

Antivirus, firewall, or other security apps may conflict with the Outlook services and processes. For instance, firewall rules may block access to certain web address or IP that may prevent it from communicating with the server or synchronizing the mail items from the OST file to the server. Similarly, antivirus, when scanning, locks the OST file and prevent any operation, such as read/write, resulting in the synchronization log issues.

You can prevent this by:

• Temporarily disabling the security apps.
• Adding Outlook as exception in security apps.
• Removing the apps if they don’t allow you make these changes or exception and installing an Outlook-aware security app.

6. Manually Sync Mail Items

When automatic mail synchronization fails multiple times or generates errors, Outlook may not update it further or stop syncing. In such a case, you can try manual synchronization. This may resolve temporary sync issues and fix the problem once initial synchronization becomes successful. To do this,

• Open Outlook and click on the Send/Receive tab.
• Under this tab, click Update Folders.
• Wait until the synchronization completes

• Once this completes, you can send a new mail and then check if it automatically synchronizes or not.

7. Setup a New Outlook Profile

Corrupt Outlook profile can also lead to synchronization issues in Outlook. In such a case, you can create a new Outlook profile. To create a new profile, follow these steps:

• Open the Control Panel and click on Mail.
• Click ‘Show Profiles’ and then click Add.

• Enter a profile name and click OK.

• Then, configure your email account and settings. Click Next > Finish.

• In the Mail Control Panel, you may choose ‘Prompt for a profile to be used,’ which will prompt you to choose a profile when you open Outlook.

• Selecting ‘Always use this profile’ requires choosing the new profile from the dropdown.

• Click ‘Apply> OK’ and launch Outlook with your new profile.

8. Rebuild Outlook OST

Rebuilding or recreating the OST file can fix integrity issues and resolve synchronization log issues in Outlook 2019, 2016, 2013, and 2010. To rebuild the OST file, follow these steps:

• In Outlook 2013, 2016, or 2019, right-click on your profile email address and choose Open file location.

• This will open the default location where the OST file associated with your mailbox profile is stored.
• Close the Outlook client and back up the OST file by copying it to another location, such as an external drive. You may also rename the OST file with the .bak extension for backup.

• After the backup, delete the original OST file from the default location. 
• Now restart the Microsoft Outlook client. This will create a new OST file at the same default location automatically.

Wait for Outlook to finish synchronizing all mail items from the mailbox server and then check your emails and ‘Sync Issues’ folder for any synchronization log issues.

Also read: How to Fix Outlook and OWA Synchronization Issues

If the OST rebuild fails or you find any missing emails or mail items after synchronization is done, you can convert the backup OST file to PST using an OST to PST converter tool, such as Stellar Converter for OST. OST file rebuild often fails when the user profile is deleted or disabled from the Exchange Server or Microsoft 365. In such cases, you can use the software to move mailbox items from inaccessible or orphaned OST files to PST. After the conversion, you can import the PST into your newly created OST file or any other Outlook account via the Outlook Import Export wizard. You can also directly export the PST to an existing Outlook profile, live Exchange Server, or Microsoft 365.

To Wrap Up

Outlook synchronization log issues can occur when Outlook fails to establish connection with the mailbox server or encounters problems when synchronizing the mail items with the mailbox server. Above, we have mentioned various methods to fix the Outlook synchronization log issues. In case the issue has occurred due to damaged OST file, you can create a new OST file after taking its backup. After creating the file, check for any missing items in the file. If there are any, use an OST to PST converter software, such as Stellar Converter for OST to extract the missing items from the backup OST file.  

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About The Author

Ravi Singh

Ravi Singh is a Senior Writer at Stellar®. He is an expert Tech Explainer, IoT enthusiast, and a passionate nerd with over 7 years of experience in technical writing. He writes about Microsoft Exchange, Microsoft 365, Email Migration, Linux, Windows, Mac, DIY Tech, and Smart Home. Ravi spends most of his weekends working with IoT (DIY Smart Home) devices and playing Overwatch. He is also a solo traveler who loves hiking and exploring new trails.