The manner in which email is used gives users the impression that messages are written by one party and then received by the other party. In reality, a third party (the email server) is in between the sender and receiver. Due to this 'middle man' email server, it has access to all of the information that the receiver has access to.
Unfortunately, an email server is necessary for email to work. When a message is sent over email, it is first sent to the server to be stored since it is too difficult to send it directly to the receiver. When the receiver of the message logs into their mail account, they ask this email server if they have any new messages. The email server then sends the new messages to the receiver.
Since the email server stores messages before they are sent to the receiver, anyone with access to the server has access to all of the stored messages. This information includes message contents, attachments and any other meta data (i.e. who it is to, who it is from, etc.)
As the world gets more and more dependent on email, it is important for people to know its security weakness. This way anyone with confidential information to send over email will look into more secure systems before sending.