My life is on Gmail. It's not an exaggeration - I was one of the first adopters of Gmail and have been using it as my primary email address since 2004. I have over 7.5GB of email in there, documenting many of my life's biggest decisions and transitions. I've been looking for a solution to back up all my email since it makes me a bit nervous to have absolutely no control over something so valuable to me.
Yesterday, I figured it out. Thanks to this excellent tutorial, I was able to set up a fetchmail daemon to download all my emails via POP3 and store then locally on my server. It's still running, but so far it has downloaded about 14,000 emails starting from the earliest one.
Next step is to set up a cron job to peridiocally take snapshots of my /var/mail/[username] file and store them on another machine (or AWS). I'll write about that process when I get around to doing it.