Microsoft OST file is the mirror copy of a user's mailbox on Microsoft Exchange Server. It is a useful file, which makes the user's mailbox data available for work when a connection to email server is broken. For every email profile, an OST file is created on user's PC. This file not only makes the data available to you but also lets you work when Outlook's connection to exchange server has broken. Whatever you do during offline mode, it will be automatically updated on Exchange Server when it has an active connection with Microsoft Outlook. During offline mode, you can write emails, edit emails, delete emails, move emails from one folder to another, and so on.
When the synchronization between Outlook and Exchange Server is done, both OST and the user's mailbox on the email server are updated to have identical copies at both places. During the synchronization, only the changes done during the offline mode are copied in the user's mailbox on the email server.
Whereas OST file is beneficial for a busy employee or anyone else, you must ensure that you do not make changes to the email account profile or the user's mailbox on the email server. Even a slight modification in the user's profile can make the OST file inaccessible. Similarly, if the user's mailbox on the server is deleted or damaged, OST file will become orphan and inaccessible. In both cases, MS Outlook will not be able to synchronize OST file with the Exchange Server, and you see an error message related to Outlook. Depending upon the version of Microsoft Outlook, you may see a different error message, but that message will clearly convey an issue about synchronization.
When you get OST synchronization error, you can perform the below steps to fix the error message:
In case, the OST has become orphan, you will not be able to access it in Outlook even after changing the email profile settings to their original state. In this case, you will have to use a smart OST to PST Converter like OST to PST Conversion tool to access data from the OST file and save it as a PST file. Once the PST file is created with all data in it, you can open it with your Outlook account. After this, you can perform the Step 2-4 above to copy data in your mailbox and have it in a new OST file.