When you work with Microsoft Outlook, you create emails, contacts, and other items in an Outlook data file, or .ost file. The .ost file is a replica of the information in your Exchange mailbox. The .ost file is stored on your computer. If your Exchange server goes down, you can still access your email by using the .ost file.
If you want to back up your .ost file, you can export the contents of the .ost file to a .pst …
