The OST file is a database file used by Microsoft Outlook. Its main purpose is to make it easier to work offline even when the connection to Exchange is not active. Due to backup Exchange Mailbox users want to convert OST to PST file format. Use the most prescribed techniques to convert OST to PST file format. You can use vMail OST to PST Converter Tool it allows users to export only the first 30 emails per folder. It converts OST files into PST and also various other file formats like MBOX, EML, EMLX, HTML, CSV, MHTML.