Step 1: Install Thunderbird and "ImportExportTools" Add-On for Thunderbird
1) Download and Install the Thunderbird mail application: http://www.mozilla.org/thunderbird/
2) Now, download the "ImportExportTools" extension utility that works with Thunderbird, go to http://www.nic-nac-project.de/~kaosmos/mboximport-en.html
3) On that page, locate the "Download ImportExportTools (MboxImport enhanced)". Right-click the link to download the xpi file and select "Save target as". Please note the location that you saved the .xpi file
4) Launch Thunderbird, go to "Tools - Add-ons".
5) Now you are in the Add-ons Manager, go to the drop-down in the upper right corner that looks like a gear and click on "Install Add-on from file"
6) Locate the .xpi file that you downloaded and follow the instructions prompted to install
7) Restart Thunderbird
Step 2: 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 3: Import the emails from the MBOX file for the label that you want to view
1) In Thunderbird, import the MBOX file to a local folder; in the navigation pane on the left, then right-mouse click "Local Folders" and then select 'Import/Export - 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 import 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 4: Create a filter to search by a specific date range
1) In Thunderbird, right click on the local folders drop down menu, select 'new folder', and give the folder a name. This will create a new folder in your local drive.
2) After the folder has been created, right click on the imported .mbox file and select 'search messages'.
3) Here you will be able to define search filters to narrow down your results. (Example: If I want to search for all messages before June 1st, 2012, I would change the search field to 'Date', the next field to 'is before' and in the next window I would type 06/01/2012.) Then click search.
4) In the section below, you will see a list of email files populate that are within that search range. Select all of these files and on the bottom of the search window select 'File' then 'Local Folders" and then select the folder you have created. This will import all of the files from the search filters to this folder.
5) This folder can now be saved as a .mbox or be uploaded back to the original email account. To save it as an .mbox file, right click the folder and select 'ImportExportTools'. Click 'export folder' and select open. This will save the file as a .mbox file on your local drive.