Go to takeout.google.com
Click the profile icon in the top right corner to make sure that you are logged into your PSI account.
Click “deselect all”.
Scroll down to Mail.
Check the box next to Mail and click "All Mail Data Included".
Scroll to the bottom of Google Takeout and click "Next Step"
Set delivery method of "send download link via email"
Set frequency to "once"
Leave file type as zip and file size as 2GB
Click "Create Export".
When, prompted enter your password.
You will get an email saying that an archive of data was requested. The page you'll see will say "export in progress". User will get another email when the export is completed. The email will contain links to the zipped archives of emails.
You have one week to download the files before the link expires.