If you're using an Outlook.com, Outlook 365, or Exchange account, this information is stored in an Offline Outlook Data File using a .ost format. However, if you're using a POP or IMAP account like those you get from an Internet Service Provider (ISP), your information will be stored in an Outlook Data File using a .pst format instead.

Here are the steps to recover:

Open Exchange Email application (Outlook) in offline mode
Click on File menu > Open & Export > Import/Export
Import & Export wizard will appear, click on Export to a file > Outlook data file (.pst)
Choose the OST folders which contain the required Exchange information and click Next button
Browse the location where you want to save the recovered data, and then click the finish button.
The messages are exported to the newly created PST. After this, you can copy and import the messages from the PST back to the server-based mailbox.

Before starting the recovery process, you may try the free methods to convert OST to PST first.

Free - https://www.inspiredatacare.com/blog/fr … erter.html