HOW TO: Import mail into Gmail with the Gmail Loader
So, you’ve turned your back on traditional mail clients and get your mail fix via Gmail these days. 
The only problem is getting to all those old message that are stuck in your old email client. One way to stuff that old mail into your shiny and capacious Gmail account is to use Mark Lyon’s Gmail Loader.
GML is GPLed and written in Python, and available for Linux, Mac OS X, and Windows. Most popular Linux distros install Python by default. You’ll also need the Python Megawidgets, which may not be installed by default. On Ubuntu and Debian, you can just grab the python-pmw package:apt-get install python-pmw
Linux.com | Import mail into Gmail with the Gmail Loader