Downloading Letter Opener
You need a Mac computer with Mac OS X, version 10.3.0 or later.
Quit Mail.app, if you have it running
Open the Letter Opener download
If a warning dialog opens, simply click Continue.

If Installer not open automatically, double-click on the Letter Opener icon.
Follow the Installer

Open Mail.app from your Applications folder

Letter Opener Registration
Letter Opener is Shareware, if you like it you can buy it and get away of the registration dialog.

After they payment was successfully you get a mail with your registration details and you can finish the registration process.