When you send an email across internet, you must first log into your ISP's email system. Generally, you set login information (username and password) in some setup screen, then quickly forget about it. However, behind scenes your username and password are used to log in each and every time you send email.
When email system receives your message it opens a connection to recipient email system and delivers message. This is way email normally works, at least on properly configured email systems.
Unfortunately, many emails systems are not properly configured (or have older, buggy software). These systems have become bane of internet and are one of primary reasons that spam is so hard to fight.
These email servers are known as open relays. You see, email systems also have ability to send messages to one another. This is known as relaying. In good old days of internet, back when it was a small network of universities and military installations, spam was not a significant issue. During those innocent times, there was little security because there were not many offenders. Thus, email systems did not protect themselves very well.
What is there to protect against? Spammers use open relay systems to hide their identity. What happens is simple. A spammer sends messages using one of these open relay systems and bypasses normal security. The spammer is basically hijacking email server to do his dirty work for him.
You see, email messages are actually enclosed in an electronic envelope which identifies where message came from. So if a spammer sends a message through his own ISP's email server, then it could be tracked back to him because he has an account on that server.
However, if he hijacks an open relay, he can send all of messages that he wants without worrying about being tracked. The email message identifies open relay as system where email came from; however, spammer is not a legitimate user. The open relay does not (unless it goes to great lengths) have a clue where messages came from.
A spammer must rub his hands together in glee when he finds one of these systems. I can just imagine evil laugh as spammer presses return key to send literally hundreds of thousands or even millions of messages through open relay system.
This cannot happen on a properly configured, secured and patched email server.
Open relays are a big problem, and to combat that problem a number of services have appeared. These are called Blackhole Lists, and what they do is simply list all of open relays that they know about. ISPs and others can subscribe to these lists and use them to block email messages.