If You want to convert or export your mails from Zimbra to Outlook then there are many tools and software are in market which but if want the reliable one I will tell that software which i have tested and used many time to convert or export mails the software name is Zimbra to Outlook Converter this will help to you convert your emails in PST and let you do that in very simple and easy way. It’s very easy to use that tool and that is secure your data too. 
Visit here more info:  https://www.spikevare.com/convert-zimbra-thunderbird-mbox/
 

Try Advik OST Converter for 100% accurate results. Using this tool you can convert ost to pst file format for Outlook 2019, 2016, 2013, 2010, 2007 etc versions. Also the tool offers multiple saving options such as OST to PST,  MSG, EML, MBOX, EMLX, RTF, Text, HTML, DOC, Maildir, TGZ etc.
Its an all in one toolkit.