How to fix sync errors on Outlook

Question

I receive many sync errors on my Outlook account including a sync log. How can I fix this? I have a lot of folders for different clients.

Answer 1

Hi Sue Svensen

Thank you for reaching out to the Q&A Forum.

To better understand your situation, could you please share some information:

• Did you see any specific error code or message from your Sync Log? Please shared this error message (Please remove any private email addresses).
• If you log into Outlook on the Web, do you see the folders/emails that are missing or erroring out on your desktop?

Since you mentioned having "a lot of folders", based on my understanding, Outlook often struggles to sync if you are downloading "All" emails for a massive folder hierarchy. You can try this:

• Go to File > Account Settings > Account Settings > Double-click your email account.
• Look for the "Download email for the past" slider > Try to change it from "All" to 1 Year (or 6 months).
• Click Next and restart Outlook.

Additionally, you can try Create an Outlook profile - Microsoft Support and restart Outlook then try again.

I hope this information proves helpful and look forward to your response!

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Answer 2

Sync errors in Outlook are usually caused by issues with the local OST file or with specific folders. Use the following steps, starting with the least disruptive.

1. Resync specific problem folders If errors mention particular folders (for example, client folders):
2. In Outlook, right-click the folder that shows sync issues.
3. Select Properties.
4. Select Clear Offline Items, then OK.
5. On the ribbon, go to the Send/Receive tab.
6. Select Update Folder.

Repeat for other folders that appear in the Sync Issues log. This forces Outlook to download a fresh copy of those folders from the server.

2. Rebuild the OST file (cached mailbox) If sync errors continue or are widespread across many folders, rebuild the OST file.

Outlook 2019/2016/2013/Outlook for Microsoft 365:

1. In Outlook, select the File tab.
2. Select Account Settings > Account Settings.
3. Go to the Data Files tab.
4. Locate the OST file for the business account and note its path and filename.
5. Close all dialog boxes and exit Outlook.
6. In File Explorer, browse to the OST path and delete the .ost file.
7. Restart Outlook and allow it to create a new OST and fully resync the mailbox.

Alternatively (any recent Outlook version):

1. Close Outlook.
2. In Windows, go to C:\Users\<username>\AppData\Local\Microsoft\Outlook.
3. If necessary, unhide folders and file extensions (View > Options > View > enable Show hidden files and folders and clear Hide extensions for known file types).
4. Rename the OST file(s) to .old.
5. Start Outlook. It will show Preparing for first use and then resynchronize the mailbox. Test whether sync errors still appear.

Important: If there is local data that does not exist on the server (for example, items moved to local-only folders), export those folders to a PST before deleting/renaming the OST, then import the PST back using Do not import duplicates.

3. Use the Sync Issues folder to verify
4. In the Folder List (left navigation), expand all folders and locate Sync Issues.
5. Review the log messages after resync/rebuild to confirm that errors have stopped or reduced.
6. If errors persist with many client folders If there are still performance or sync problems, especially with many folders or secondary mailboxes:

• Consider whether a secondary mailbox or shared mailbox with many folders is involved; performance and sync issues are more likely in that scenario.
• In that case, reducing the number of folders or items in heavily used shared/secondary mailboxes can improve synchronization reliability.

If, after rebuilding the OST and resyncing folders, the Sync Issues log still shows new errors, the problem may be server-side and should be escalated to the Exchange/tenant administrator for further investigation.

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References:

• Synchronization problems occur in Outlook and in Outlook Web App
• Synchronization issues between Outlook and Outlook Web App
• Terminated in error found in Outlook Sync Issues log files
• Performance and synchronization problems when you work with folders in a secondary mailbox in Outlook