How to use Gmail to do spam filtering and mail backup

Here is a clever method of using G-mail as your spam filter and mail backup. Especially with the current scheme of infinity+1 program, there is no reason why people should not be backing up their e-mail with this method. Not only do you have a complete spam-filtered backup, but it’s free!
Thank you G-mail!
This requires you to run your own mail server, and know how to configure the mail server properly to forward via header settings.
Via MBoffin