Think of e-mail as a postcard. The postcard will get to its destination, but any postman along the route can read it.
When you send an e-mail message, you are sending a text file that contains routing information - From, To, Date, Subject - and your message. When you hit "Send," the file is moved to your e-mail server and then transmitted to an Internet mail router where the file "asks directions" to the destination. The router compares the mail destination to a list of locations and sends the message to the next available router until the message finally finds the recipient’s e-mail server. While this is happening, your e-mail can be read by anyone with access to one of the servers your e-mail passed through.
The only way you can secure an e-mail message is to encrypt the body of the message. Encryption is a way of scrambling the contents on your "postcard" in a way that your recipient can decode but that no one else can figure out. Encryption has to be set up ahead of time, though. You and the recipient have to have the same "code-book" or you won’t be able to decode each other’s messages.
Unless you are using strong encryption, you should always assume that your messages can be read by an outsider, and avoid sending sensitive or confidential information in an e-mail message.