For historical reasons, many SMTP mail servers will accept mail for email domains other than their own, and forward it on to the intended recipient. Third-party relay, also known as "open relay" or "insecure relay" is where a mail server will route mail for anybody in the world. Any machine which will accept mail for any domain and forward it on regardless of who the sender is or what IP address the mail is sent from is generally called an 'open-relay'.

Spammers hunt for and abuse these servers to try to cover their tracks, because they know their spam is unwelcome and unwanted. When spammers locate such a machine they can use it as a free distribution service for their junk email. This process very often leads to your IP/domain being blacklisted.

A whitelist is a list of emails, domains or IP addresses from which you will always accept email, even if it would otherwise be considered spam. For example, newsletters you subscribe to.

A blacklist is a list or of emails, domains or IP addresses from which you will never accept email even if it is otherwise legitimate.

There are two types of spam filter, one that blocks known spam and another that blocks new or unknown spam. Many anti-spam companies like Symantec Brightmail use honey pots, or fake email address to collect spam and create filters to stop that spam. MessageLabs uses Brightmail as one of its filters.

For unknown spam, a heuristic algorithm is often used. It checks the structure of the email itself, how many people it is sent to, if it has links, phone number, and so on. Based on these factors, the algorithm decides is this email is spam or not. MessageLabs uses Skeptic to identify such spam.

DOS is an attack designed to bring the network down by flooding it with requests and traffic. A mail or web server can only handle a finite amount of requests and will crash once that limit is reached.

A variation on DOS is Distributed DOS, where a few to thousands of computers at various locations are part of a coordinated attack on a network.

It is a type of program that is often confused with viruses. Trojan Horse is not a virus, but simply a program (often harmful) that pretends to be something else.

For example, you might download what you think is a new game; but when you run it, it deletes files on your hard drive. Or the third time you start the game, the program e-mails your saved passwords to another person.

There are two types of anti-virus filters, one that blocks known viruses based on their signature and another that blocks new or unknown viruses based on behavior. Many Anti-Virus companies use a combination of commercial engines to increase their chances of stopping a virus.

The difficult part is stopping new and unknown viruses. A heuristic algorithm that checks the structure and behavior of the email attachment is a popular approach, although it takes years of data-acquired experience and expertise to create one that works well.