Resolve "Unable to Expand the folder" error while trying to open Outlook

Outlook OST file synchronizes every time you get connected to the Exchange server to update the local ost file and the Exchange Server. Sometimes this synchronization fails that results in inaccessibility of the ost file. Suppose a scenario where in you are trying to open MS Outlook but it fails to open or synchronize the offline storage file i. e. OST file, and reflects following error message. "Unable to expand the folder. The set of folders could not be opened. Errors could have been detected in the file xxx.ost. Quit all mail-enabled applications, and then use the Inbox Repair Tool." xxx.ost here is the offline storage file (OST file) created by MS outlook on your local system. Showing this error message means the ost file gets corrupted and it can not be synchronized with the Exchange Server. As the ost file can not be directly opened with Outlook, the ost file becomes inaccessible. Resolution: Follow the below steps to resolve the situation 1. Close MS Outlook and all other applications that may access the OST file. 2. Find the respective OST file, that causes the error. Information in the error message contains the ost file name. 3. Backup the original OST file. 4. Convert Outlook OST to PST file using third party ost to pst converter. On successful conversion of ost to pst file all your previous emails and data are now safe in an Outlook accessible PST file. 5. Rename or delete the OST file that causes the problem. 6. Make sure the email account settings in Outlook are correct. Restart Outlook and send/receive your emails on the corresponding Exchange mailbox, Outlook itself create a new OST file and synchronize its data with the Exchange mailbox. If in case you can not access your mailbox, then your current mail profile is incorrect, delete it and create a new one, following these steps: Close MS Outlook. Go to The Control Panel Double-click Mail. In the Mail Setup dialog box, click Show Profiles. Select one of the incorrect profile in the list and click Remove. Repeat the above step until all incorrect profiles have been removed. Once all the incorrect profiles have been removed create a new profile following these steps. Click Add and add email accounts properly according to their settings on the server. Start Outlook and re-synchronize your Exchange mailbox. 7. Import recovered Data : After your OST file problem is solved, Open the new ost file and converted PST file that contains all the recovered data, copy required data from ost 2 pst file.