Open Source Tool To Backup Gmail Accounts: Gmvault
Gmail is probably the most widely used e-mail provider between creative people. It is always simplistic, modern and always works.
Personally, besides my personal e-mail, I use their "custom domain" feature for handling the e-mails of all my projects.
As expected, the content there is very valuable and hard to risk losing. Gmvault is an open source tool that saves the day by backing up Gmail accounts to your drive smartly.
It can backup the entire Gmail account at once and can synchronize afterwards by only picking the non-backed-up ones (there is also an optional encryption).
If we ever want to restore them, Gmvault can do that as well. They can also be restored to another Gmail address (including labels).
There is a Windows installer and ashell/batch client exists for Linux, Mac OSX and Windows.