The way typical email encryption works is that you have a public key and a private key (this sort of encryption is also known as Public Key Infrastructure or PKI). You, and only you, will have and use your private key. Your public key is handed out to anyone you choose or even made publicly available.
If someone wants to send you a message that is meant only for you to see, they would encrypt it using your public key. Your private key is required to decrypt such a message, so even if someone intercepted the email it would be useless gibberish to them. When you send an email to someone else you can use your private key to digitally "sign" the message so that the recipient can be sure it is from you.
See <a href="http://netsecurity.about.com/cs/emailsecurity/a/aa051004.htm">here</a> and <a href="http://www.pgp.com/products/freeware.html">here</a> for more info.
If you want to send a message to someone and NOT let them send to someone else then this is not realistic. There are products which advertise that they can do exactly this but they cannot. If the user can see something, they can always take a screen print of the information and then forward that image.
If you want to keep your information away from others until the recipient receives it, then follow the links above to learn more about encryption.
I would suggest using Voltage SecureMail through the Voltage Security Network (or VSN). I use it in Outlook and it's super simple (but you can use it with other apps or on the web too). With the technology, they're able to use email addresses as the public keys, so you don't have to spend time lookng them up each time you want to send a message. And you can even send encrypted messages to people who have no software, which makes it even better. Find out more at http://vsn.voltage.com/