I recommend using KDETools Zimbra to MBOX converter. This tool converts Zimbra files to MBOX without any hassle. With this software, you can convert multiple Zimbra files to MBOX at once. It converts Zimbra files to MBOX, PST, OLM, EML, EMLX, HTML, and more. It supports all MBOX mail clients.
Read More: https://www.kdetools.com/zimbra/mbox/
To convert TGZ to MBOX, you can use third-party operating systems. However, if you want the simplest solution, use the SameTools TGZ to PST Converter tool. This tool simplifies the TGZ to MBOX conversion and saves you time and effort. You can also use this advanced utility to convert the TGZ database to MBOX, EML, EMLX, MSG, and other formats. The tool supports Windows 10, 8.1, 8, 7, Vista, and XP.