Using GPG with popular email software
If you are using Evolution, you should compose your message as you normally would and then select PGP Encrypt from the Security menu in the message composition window. Once you do this, Evolution will query the keyservers. If Evolution is able to find a key that matches the email address of the recipient, your message will be automatically encrypted as it is sent.
Kmail and Thunderbird handle encryption in much the same way. Unlike Evolution, Kmail or Thunderbird will not automatically query the keyservers to match the recipient email address with, but will only allow you to use the public keys you have installed on your computer. After you have chosen a key, the message will be encrypted with it and sent. While this encryption process is not as automated as Evolution’s, it does prevent the small possibility of automatically encrypting with an old or expired key that the recipient no longer uses.
Source: ERM Blog