Understanding digital signature and email
 

Hello,

Please help me to understand digital signature a little better. To sign a document digitally (ie: an email), you need a private key that you keep for yourself, and a public key that you give away, correct? So, when I receive an email with a public key, I don't understand why I couldn't forward the same email to some one else, with the same public key, in the name of the original sender... It would still be signed by him because it's not encrypted, and it has the public key correct? Then I don't know why having a private key... Shouldn't an email sent digitally signed be encrypted along with the public key? Why aren't they, then? I'm sure many of you have received these kind of emails, especially by system adninistrators who like to sign their mails.
Please let me know. Thanks.

This will explain it better than I can:

What does "BEGIN PGP SIGNED MESSAGE" mean?
http://ask-leo.com/what_does_begin_p...sage_mean.html

This might help you understand them:
http://www.youdzone.com/signature.html
Or my favorite - HowStuffWorks.com...
http://computer.howstuffworks.com/question571.htm

Thanks guys.