A very common issues with using Outlook is the Synchronization Between OST and Exchange Server.
Here at Ibmi Media, as part of our Server Management Services, we regularly help our Customers to fix Ubuntu related issues.
In this context, we shall look into the causes of this Outlook error and how to troubleshoot it.

Why is it important for Synchronization between OST (Outlook data file) and  Exchange Server?

Synchronization of Exchange mailboxes with their offline counterparts (on user machines) is essential for stress-free offline working in the Exchange environment.
Outlook application synchronizes OST file with Exchange Server mailbox when Exchange connectivity is there.
It is likely that you will encounter error messages or face issues when the synchronization of their OST file (Offline Outlook Data File) fail on Outlook.

What are the causes for synchronization issues?

Synchronization issues between OST and Exchange Server can sometimes happen due to bad network connection or hardware defects and the synchronization can fail fully or partially.
Also, the failure of Exchange OST Synchronization can also happen as a result of corruption or damage of OST files.

How does Exchange OST Synchronization errors appear?

Exchange OST Synchronization issues are:
1. At the time of Exchange synchronization, we get an error message:
Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened. The file xxx.ost is not an Outlook data file (ost).
2.  When Cached Exchange Mode is enabled it can lead to synchronization issues between OST and exchange server.

All versions of Outlook are affected by OST synchronization issues, including Outlook 2019, 2016, 2013, 2010, 2007 & 2003.

How to enable Automatic synchronization?

For the automatic synchronization, we can follow the below steps:
1. Click Tools, then Options.
2. Under the Mail Setup tab, go to Send/Receive tab and click the checkbox for Send immediately when connected.
3. Then click Send/Receive.
4. After that check that ‘Perform an automatic send/receive when exiting’ option is clicked under the Send/Receive Groups dialog box.
5. Finally, give OK.

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Methods to fix OST Synchronization issues?

1. OST Integrity Check Tool
OST Integrity Check tool (Scanost.exe) is for Outlook 2003 and Outlook 2007 versions. This tool is not useful with the recent versions of Outlook.
2. Manual synchronization of offline folders (for Outlook 2010, Outlook 2013, & Outlook 2016).
When we face synchronization issues with specific folders we can try this method to fix synchronization issues at the folder level.
Steps to follow are:
a. First Right-click on the OST folder.
b. Then click Properties.
c. Click on Clear Offline Items and click OK.
d. Finally, click the Update Folder button in the Send/Receive tab.

3. How to Rebuild the OST file?

If the OST issue persists, we can rename the affected OST file and let Exchange rebuild the OST file.

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How to rebuild the OST file?

We can rebuild the OST file to fix synchronization issues between OST and Exchange Server.
Steps to follow are:
1. First close the Outlook application.
2. Go to the location C:\Users\administrator\AppData\Local\Microsoft\Outlook.
3. Find the OST files (For example Outlook.ost, Oultook0.ost, Outlook1.ost, etc.) and rename them.
4. Restart Outlook, and wait till the mailbox gets synchronized.

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