If you have recently exported your Gmail mailbox to an MBOX zip file, here are the instructions to import your MBOX into a new gmail account.
Step 1: Set up your Gmail account with Thunderbird
1) Download and Install the Thunderbird mail application: http://www.mozilla.org/thunderbird/
2) Launch Thunderbird and go to Create a new account: Email
3) Click “Skip this and use my existing email”
4) Enter your name, Gmail email address, and Gmail password and click “Continue”
5) Make sure IMAP is selected and then click “Done”
Thunderbird will automatically get your settings if using Gmail
6) Now you will see your Gmail account in the pane on the left-side of the screen and you can open it to see all your emails.
Thunderbird is now properly set up for your Gmail account
Step 2: Install “ImportExportTools” Add-On for Thunderbird
1) To download the “ImportExportTools” extension utility that works with Thunderbird, go to https://addons.mozilla.org/en-us/thunderbird/addon/importexporttools/
2) On that page, locate the “Download Now”, ensuring the download supports MBOX files. Save the file, noting the location where you saved the .xpi file
3) In Thunderbird, go to the menu in the upper right corner and and click on “Add-Ons”
4) In the Add-ons Manager tab that opens, go to the gear icon, then “Install Add-on From File”
5) Locate the .xpi file that you downloaded and follow the instructions prompted to install
6) Restart Thunderbird
Step 3: Extract the zipped up email archive file provided to you
1) Download and Extract the Gmail Export that we have sent you. The archive file is in .ZIP file format.
2) Extract the .zip file that you received. Once extracted, you will have several MBOX files which are each named according to the corresponding label in Gmail. Please note the location of where you extracted the files to.
Step 4: Import the emails from the MBOX file for the label that you want to restore
1) In Thunderbird, import the MBOX file to a local folder; in the navigation pane on the left, right-mouse click “Local Folders” and then select ImportExportTools > Import mbox file
2) Choose the option to “Import directly one or more mbox files” and select ‘OK’
3) In the files browser, locate the folder where you extracted the mbox files to in step 3 above. Select the mbox file that corresponds with the label that you want to restore and then click ‘Open’
4) All emails from that label will now be importing. Please be patient, as it will take a while to load all of the emails.
Step 5: Copy the imported emails in to your Gmail Account
1) In Thunderbird, in the navigation pane on the left, expand “Local Folders” and then select the folder below that has the name of the label that you are importing to Gmail
2) In that folder select all of the emails contained in the folder, right-mouse click the selection, and select ‘Copy to – <gmail_account> – <label_name>’. (where gmail_account = the email address of your Gmail account and label_name = the path to the label that you want to import the restored emails to)
3) All emails will now be importing to the label that you choose. Please be patient, as it will take a while to upload all of the emails to Gmail.
Once this is completed then you can log in to your Gmail account online and you should see all of the emails restored to the label that you chose in your Gmail account.