Integrating PGP within the email client is important. While one might use their public key program to generate a signed version of a document (gpg --clearsign file) and send that as an email... it is a hassle few of us would advocate.

Fortunately, most email clients have been given the ability to handle most of the nuances of signing an email message, so that all you have to do is periodically give it your passphrase, and it will sign messages for you. This page provides links for how to set that up

The commercial PGP supports the major commercial email packages, including most versions of Outlook, Outlook Express, Eudora, and Apple Mail.

The free GPG has projects to support most of the major commercial and the free email programs. Support is stronger with the free programs, like Netscape/Mozilla, and sometimes it is even provided by the mail developers, in the case of Evolution and mutt. The GnuPG Front End page has a set of links for your particular email client of choice.