In Webmail, when viewing the message you would like to forward, click on Forward as Attachment. (To view but not forward the full headers, select View Full Header under Options:.)
In pine, type H to display full headers and then F to forward the message. Type N when given the option to forward as an attachment. (If typing H does not display full headers, you need to enable full headers. To enable full headers, type M, then S, and then C to get to the setup configuration. Type W and then full-header and Enter to locate the enable-full-header-cmd option. Type X to set the option, followed by E and Y to exit.)
In OS X Mail, if you have a Show Headers icon in your toolbar, simply click on it before forwarding. Otherwise, go to the View menu and select Show All Headers before clicking on the Forward icon.
In Mozilla, go to the View menu and select Headers and then All. Then go to the Message menu and select Forward As and then Inline to forward the message.
In Eudora, click on the BLAH BLAH BLAH button on the upper left hand corner of the message. You may then have to cut and paste the message into a new email.
For information about how to view full headers in other email clients, see http://spamcop.net/fom-serve/cache/19.html. Additionally, Outlook users may find the plugin at http://www.daesoft.com/freeware/spamsource/ useful.