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 Zmbra to PST and various file formats such as EML, EMLX, HTML, PDF and etc. 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/
Steps-1: Open Zimbra desktop on your operating system
Steps-2: Go to the 'Preference' tab from the Menu Bar.
Steps-3: Click on the 'Export or Import' button.
Steps-4: Go to the 'Advanced Settings' tab and select the files that you need to export.
Steps-5: A pop-up window will appear, asking you to save the .tgz file at the desired location. Go to the 'Save File' option.
Steps-6: Click on TGZ file and right-click on the button to extract the files.
Steps-7: After unzipping file successfully, you will get a folder with the same name as shown here: Click on 'OK'.