E-mail is insecure by default because it is more like a postcard, not a sealed envelope
A number of people are under the misconception that when they draft and send e-mail, two things occur. Their message gets sealed in an envelope (that's why you have to open e-mail right?) and that it goes directly to the person it was sent to via Internet magic.
The truth is your e-mail is sent in plain text (i.e. readable by anyone who picks it up along the way) and is passed around the Internet with multiple stops until it reaches its destination. People with evil intentions can intercept your e-mail, read it or even alter it before it reaches your intended recipient.