There are many third party software available on the internet but to select best software is difficult. I personally suggest KDETools Zimbra to Outlook Converter. This software easily convert Zimbra to PST and various file formats such as EML, EMLX, HTML, PDF and etc. This software maintains the hierarchy of folder structure. It also comes along with a free trial version which allows you to see the preview of all convertible emails.
Read More:
https://www.kdetools.com/zimbra/
