If you already have email in outlook you can skip these steps

  1. Setup Thunderbird to get mail and connect to your Gmail using IMAP – instructions here
  2. Import the PST file to your Outlook

Import emails from outlook to Thunderbird

  1. Go to Thunderbird

  1. Click Tools -> Import -> Select what you want to import

  1. You should now see your mails in your Thunderbird by now,