Many users used Microsoft Outlook and this application create two type of file such as OST file and PST file. Both files are store mailbox data into your system but OST fiile create on Exchange Server Catch Mode and PST file is create POP3 & IMAP. Some time OST file curropt some technical issue such as System failure, virus Attach, Account deteted, Windows curroption and Server Downtime. in this case OST FILE Not OPEN in Outlook. in this secuation i suggest you Convert OST file into PST file.
In Conversion of OST to PST file you can try free manual method and third Party tool:
Explain manual method to Convert OST to PST File,
Step-1: Run or Launch Outlook and select to file tab.
Step-2: In file tab you select Import & Export option then open new Wizard.
Step-3: In this Wizard select Export to a File and click Next Button.
Step-4: Select to Outlook data file and Click Next after that Select a Mailbox with check Subfolder Clik Next.
Step-5: Declear a saving path to save Backup of OST mailbox data into PST file format then Click Finish Button.
This Method only work to Connected Orignal Account mails, other wise you can convert OST file into PST file using third Party tool.
I Suggest you try to one of the best OST to PST Conversion tool. ATS Software Company launch new version of OST Converter tool to repair and fix Outloook errors with recover Lost mailbox data. this software save recover OST mailbox data into Outlook PST, MSG, EMLX, EML, MBOX, HTML, vCal, vCard, and cloud apps office365, Live Exchange Server etc.