Before describing the attack itself lets first define what an ISP does. Many think that their ISP just gives them access to the Internet - a phone number to call so you can hop on the net. An ISP does that. However, your ISP provides you with a host of Internet services, some of which may be almost invisible to you. Your ISP provides you with access to Internet services such as the newsgroups, Internet mail, and others.
Lets take a closer look at Internet mail.
Sending and receiving mail via the Internet requires several components. You must have software on your PC to perform the mail functions. You must have a service provider that gets you on the Internet. You need a mail server - which is a computer that functions as a platform or holding area for your mail until you log into it to pick up your mail and a computer that you use to send mail to others. These functions may be performed by one server or two servers.