SPAM: Blacklists & Whitelists

Spam. Anyone connected to the Internet has heard the word. Anyone who has an e-mail account dreads the word. It has been estimated recently that one third of all e-mails transferred from computer to computer are unsolicitated: i.e, spam. But rather than talking about the generalities of spam this article attempts to inform you of two methods used by companies to combat spam and how they may affect you. These two methods are called Blacklists and Whitelists.

As the name suggests Blacklists are lists which exclude something. In the case of spam Blacklists exclude IP addresses which are associated with spam and block all e-mails coming from those IP addresses. But how does a Blacklist affect you? Before you can determine how this affects your web site you must understand IP addresses and how they work with web hosting.

If you have shared hosting on a server, chances are you are also sharing an IP address. An IP address is the numerical representation of where you server is. While humans use alphabetical names to remember a web site; computers and servers don't know where www.damonhost.com is but rather know that it is located at 64.191.62.74. The numbers 64.191.62.74 is the IP address of the web site www.damonhost.com. Whenever a computer needs to contact www.damonhost.com or send mail to www.damonhost.com it knows that the IP address is associated to that domain. Now, www.damonhost.com is not the only domain (or hosting account) using this IP address. With the aid of server software a single IP address can be reused by more than one domain at a time. This is called virtual hosting. There are many reasons why several domains may be using the same IP address but realize that tens or possibly hundreds of domains may be sharing the same IP address on a server. Now let's examine why this is important when talking about Blacklists.

When a spammer sends out spam, either through a hosting account, or through exploiting a security hole in a server the e-mails are sent from a hosting account which has an IP associated to it. When other computers, and users, find out that they have received spam and they trace the unsolicited e-mail back to the domain which sent the spam. What happens next is that this IP address, which is associated with the spam, is now listed as sending spam and is listed on a Blacklist. What happens next depends on the people running the Blacklists the IP was just listed on.

Blacklists can be characterized as three different types: the temporary, the permanent and the obscene. Before the differences are discussed let's talk about what a Blacklist does: a Blacklist is nothing more than a list of IPs in which all e-mail originating from them is blocked. Any e-mail coming from a Blacklisted IP address is returned to the sender without ever reaching the intended recipient. Now the computers which are using the Blacklists do not determine what is spam and what is not spam and only block the spam--they block all e-mail whether it is spam or not.

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