Exchange Mode, if you are connected to Microsoft Exchange Server and Outlook
will start creating backup of your mailbox in a cache file. This OST file can
be accessed even in the absence of synchronization.
You can also see this link: https://support.office.com/en-us/article/Open-or-import-items-from-an-offline-Outlook-Data-File-ost-a584963e-a66d-4d9d-8845-31cb270f6880
If you are unable to configure, then you should use third party application: http://www.microsoft-ost-to-pst.com/
It may help you to export your .ost file to Outlook Express eml file format.