Zimbra Mailbox provides backup in .TGZ file format which the user cannot open in the Microsoft Outlook platform. Therefore, the user needs a professional application tool to convert and transfer data from Zimbra to Outlook PST format.
We highly recommend you download the MailsDaddy Zimbra Converter tool to export data from TGZ to PT format. It manages all folder structures, headers, inline images, contacts, calendars, tasks, etc. in a new PST file. The free demo version of the software converts up to 20 emails from each folder.

Read more: https://www.mailsdaddy.com/zimbra-converter/