If you want to migrate Zimbra to MBOX then you can try an automated tool like Zimbra Converter. The application allows the user to directly migrate Zimbra to MBOX and other file formats such as EML, EML, HTML, PST, NSF, OLM, NSF, and etc. The application support 20+ email client such as Thunderbird, Seamonkey, Apple Mail, Entourage, Eudora, Opera Mail, and etc.
If you want to export TGZ to MBOX then you can try Zimbra Converter. Zimbra converter safely export TGZ file to MBOX and various file formats such as MBOX, EML, EMLX, PDF, NSF, OLM, and etc. The software maintains metadata of email during the conversion process. The application allows the user to check the preview of the email. know more information about this software visit here:- https://www.wholeclear.com/tgz/mbox/