There are a number of ways to encrypt or read encrypted email. This one is about the easiest to get installed and running on your Macintosh computer, that I've run across. It uses the native Apple Mail program, and adds a OpenPGP Encryption and Signature option.
All you have to do is install the program from the dmg file, and enter a password. There's a GUI key interface for importing existing keys into it.
Of course not all emails need encryption, but that OPSEC sensitive email you need to send to loved ones or group members is a perfect example of when to use it. Once installed, you choose what gets encrypted.
Application: GPGTools (Developed by the GnuPG group.)
Main Web site: http://www.gpgtools.org
Include in the install program are the following (from their web site):
Compatible with OS X Lion.
All applications are 64-bit compatible.
Integrated GPGMail (OS X 10.5 to 10.7, Universal).
Integrated GPG Keychain Access (OS X 10.5 to 10.7, Universal).
Integrated GPGServices (OS X 10.6 to 10.7).
Integrated GPGToolsPreferences (OS X 10.6 to 10.7).
Integrated MacGPG 2 (OS X 10.5 to 10.7, Intel).
Integrated MacGPG 1 (OS X 10.5 to 10.7, Universal).
Integrated Enigmail (Thunderbird 3 to 8).
There's even a screen-cast of the install, encrypting email, and using the Apple 'Services' feature for text edit encryption, if you want to watch it before installing: http://www.gpgtools.org/screencast.html though I'll warn you: it goes by so fast you should be ready to hit the pause and rewind buttons when you start it.
Steps [with Apple Mail closed]:
1. Download the GPG dmg file.
2. Have a password in mind
3. Open the dmg by double clicking the file in your web browsers Downloads window
4. Double Click the GPGtools.mpkg file and select an install location
5. When asked enter your email address, and name.
6. When asked, enter a password, then re-enter it when asked.
When completed, you can close the GPG Keychain Access application and start your Apple Mail.
When you select a 'new' email, you will see an OpenPGP section under the "from" drop-down list. Also you can get to the encryption/decryption options under "Messages -> OpenPGP" in your menu bar. This will allow you to sign and encrypt and decrypt your email.
In addition, this bundle of GPGTools works with Apple's Services, allowing for encryption of 'Services' aware applications.
If you open your System Preferences -> Keyboard you can click on Keyboard Shortcuts -> services and click the OpenPGP items under "Files and Folders" along with "Text" allowing you to encrypt any text file you open with textedit.
When you open textedit the next time you will see "Textedit->Services->Open PGP" in the menu bar.
The toolkit also comes with a command line interface for encrypting just about any type of file you want, but that's a little out of scope here.
For more information on the CLI, using public key servers, and general GPG information, check out this set of How-Tos.
Hope this helps, - Robert X.