You want to send and receive encrypted email conveniently with the Evolution mailer from Ximian.
Under Inbox/Tools/Mail Settings/Other, make sure "PGP binary path" refers to your encryption program, usually /usr/bin/gpg.
In the Evolution Account Editor, set your Security preferences, including your default GnuPG key, whether you want all messages signed by default, etc.
Compose an email message to someone whose key is in your GnuPG public keyring. You must trust their public key [Recipe 7.9] or encryption will fail.
From the Security menu, select PGP Sign, PGP Encrypt, or both. (Or do nothing, and the defaults you set in the Evolution Account Editor will be used.)
Click Send. Your message will be sent encrypted or signed as you requested. (You'll be prompted for your passphrase before signing.)
Evolution supports PGP, GnuPG, and S/MIME out of the box.
The home page for Ximian, makers of Evolution, is http://www.ximian.com.