Download and install Thunderbird. From the Tools menu, choose Add-ons. In the Add-ons search field, enter ImportExportTools. Click the Install button next to the add-on that appears. Thunderbird will download and install the add-on. Click the Restart Now link to restart Thunderbird.

From the Tools menu choose ImportExportTools -> Import Mbox File. In the sheet that appears, be sure that the Import Directly One Or More Mbox Files option is enabled, and click OK. In the navigation window that appears, locate your mbox file of archived mail, click the Open button, locate the file called mbox within, and click Open again. The contents of the mbox will be added to Thunderbird.

Now, when you need to read old mail, simply fire up Thunderbird and use it to do the job. Your regular email client will thank you.