E-mail is a completely insecure communication method. E-mail is less secure than normal (snail) mail. The security of e-mail can be visuallized as follows:
Imagine you and a friend are on opposite sides of a room full of strangers. You want to send your friend a message, so you write the message on a postcard. You pass it to one of the strangers standing nearby and ask him to pass it down to your friend. The stranger hands it off to another stranger and tells him to pass it down. Eventually it makes it to your friend. Or maybe not.
What can go wrong?
Anyone that handles the message can:
- read it.
- change it (including sending a completely different message to your friend, while pretending to be you).
- reply to you, pretending to be your friend.
Most of the security issue with standard email can be solved with Email Encryption.