Along the way clever and curious system administrators or hackers can read your message. The best analogy we can give is to think of your email as a postcard that you drop in your corner mailbox. The next day your mail carrier picks it up and brings it to the nearest local post office. From there it is delivered to another post office where another postal carrier delivers it to your friends' residence. Or a dishonest person could 'steal' the mail along the way.
Most network administrators have the ability to view any files on their systems, including email.
However, most don't use or abuse this privilege. That password is usually stored in a giant password file so you have access to your 'private' account when you log in. OK, so maybe we are going a bit overboard with this. After all, who would want to read your email. No one really cares about the vacation plans you are making with your brother in Little Rock.