I recommend you use MailConverterTools OST to PST Converter for your email conversion without using Outlook. This tool is a top-notch tool with advanced features. You can convert one or multiple email files to different formats and easily move your emails to services like Office 365, Gmail, and Yahoo Mail. It also helps you get rid of duplicate files and has handy features like an advanced search and organized email folders. The best part is that it works on both Windows and Mac, and there's a free version to try. It's a great choice for simplifying your email tasks. Moreover, you can also read a blog on Convert OST to PST for more info regarding how to convert OST to PST and their step-by-step process.

Getting frustrated with finding out the best professional tool to convert OST files to PST then I would like to recommend you use the ConverterTools OST to PST Converter tool. It allows to conversion of OST emails to various types of file formats such as PST, PDF, EML, MSG, MBOX, etc. without losing the data. Also, it includes the option to import OST data to numerous email clients for example Gmail, Yahoo Mail, Thunderbird, Office 365, and so on with attachments. Further, it offers the preview option to view all OST emails before taking conversion action. 
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