Use your local mail client's import functionality to import your MBOX archive.
Note: Additional information regarding the archival of your personal data can be found here.
Option 2:POP Download
Go to http://its.yale.edu/how-to/change-your-yale-passwords and scroll down to EliApps in order to set an EliApps password for your local mail client. (If you have already set this password because you are accessing your EliApps mail in a desktop or mobile client, you can skip this step if you remember the password. If you do not remember the password, you can reset it, but remember that the new password will need to be updated in your other devices.)
Log in to your EliApps account and click the gear icon in the upper right, then select Settings.
Click Forwarding and POP/IMAP.
Under the POP Download section, enter the following:
Status, select Enable POP for all mail (even mail that's already been downloaded).
When messages are accessed with POP, select keep Yale University Mail's copy in the inbox.
Next, you will need to configure your desktop mail client, which will then contain your email archive. Visit http://mail.google.com/support/bin/answer.py?answer=12103 and select I want to enable POP. Scroll down and select the local mail client you want to setup. Follow the Google instructions to configure a local mail client so that it can download your EliApps email.
Use the following Yale specific information when going through the Google instructions:
Username: email@example.com (enter your standard Yale email address)