This is something Ive set up myself, recently, to send mail through Gmail without having the unencrypted e-mail stored on their servers.To achieve this, youll need a Google GMail account, PGP or GnuPG, Mozilla Thunderbird, and the Enigmail extension.
First, set your Gmail account to allow POP3 access. This can be set in your mail settings within the web interface. The Gmail system will tell you the settings you need to make in Thunderbird in order to use this.
Next, get Thunderbird and the Enigmail extension, and install both, along with PGP or GnuPG. Then, enter your account settings into Thunderbird, as per the Google page.
Youll need to create a PGP key associated with your e-mail address. In PGP, do this using the GUI interface. With GnuPG, type gpg gen-key and follow the instructions. You can set the key type, key size (Go with at least 2048 bits. Many people use 4096) and the expiry date. Some people set their keys never to expire, I like a key duration of 6 months, so that I end up recreating keys twice a year, but at least if someone breaks or otherwise acquires my key during that time, they wont have long to do it, nor to use it, before it gets...