To import Outlook OST files into Zimbra Desktop, try the TrustVare OST to TGZ Converter Tool. This tool converts bulk and large OST files into the Zimbra.tgz file format. Without any data loss, this application transfers all items of the OST files to the TGZ file format. Users can see the preview before the conversion process. New users can easily use this application for safe conversion. Supports all Windows OS and MS Outlook versions. A free trial version of the utility.
For more information, visit here: https://www.trustvare.com/ost/tgz/