You can now download your Gmail and Calendar data in the MBOX format for backing up or moving to another service.
You can choose to download your entire inbox or narrow it down to particular labels — the same goes for calendars; pick which ones you’d like or select them all. And if you want a thorough documentation of your history across Google products, you can also download an archive file that includes your data across Calendar, Drive, Gmail, Google+ and YouTube.
The ability to download your Gmail messages will be rolled out over the next month while Calendar data is available to download for everyone today.
More info: http://gmailblog.blogspot.co.uk/2013/12/download-copy-of-your-gmail-and-google.html
