You might want to import already received mails into your gmail account, one reason could be that you want to make the complete switch to gmail and never use the old email client again. There is unfortunately no import function to do that in gmail, at least none that I was able to find in my gmail account.
Gmail Loader is the script that you need to do just that. It is a graphical cross-plattform tool that makes it possible to import mail from many email clients including Netscape and Thunderbird. Gmail Loader forwards your mail to your gmail account, pretty simple concept don’t you think ?
You can download a windows version which does not require python and a linux / osx version which requires python and is distributed as source only. The author suggests that you use the command line tool readPST to import mail from Outlook to your Gmail account. Not sure how this is working, it could be that the tool converts the outlook format into a format that is readable by gmail loader.
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I’ve used this before and it works very well. I used to this to import years of Outlook emails from a former job.