Switching from a proprietary service like Gmail to one of the more transparently-run email services on PRISM Break is the first step to a secure email account.
The second step is getting you and your contacts to encrypt your plain text messages with PGP encryption. This section contains free email clients that support PGP.
Read the Email Self-Defense guide by the Free Software Foundation to learn how to encrypt your email messages.
Here is a guide by Security In A Box to encrypting your email with Mozilla Thunderbird, GNU Privacy Guard (GPG), and Enigmail.
Find out more about the differences between Mozilla Thunderbird and Icedove.