Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.
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If that works, select all emails from the old POP3 PST file, and copy it to the IMAP folder. You can detach the old PST file from Outlook after you're done.