There are so many ways to backup emails from Outlook OST to PST but I want to tell you the easiest method by which you can backup your email. I suggest you use the
SysInspire OST to PST Converter tool. This software converts your OST file into PST and also several other formats like Zimbra Desktop, VCF, MSG, PST, IMAP Account, EMLX,EML, HTML, Office 365, and many other formats. This software also provides the free version to the user in which you can convert 25 items in each OST folder.
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backup emails from Outlook OST to PST