You will need a mbox extract tool (there are some over there, just google for mbox extract). There is even a plugin for TotalCmd to read mbox files in your windows box just with TotalCmd (
http://www.totalcmd.net/plugring/mbox.html)
Or you can replace the inbox file in any other email account with the one you just untared. Then access that email account with your mail program, and there will be user's emails:
Create a new mail account in some hosting of yourself. Now, replace the emtpy new created inbox file with the one you untared. Read the mail using that account.