A blacklist, as the name implies, is a list of people or companies who have met with the disapproval of others. In the online world a blacklist refers to those people who have been marked as responsible for generating spam in a very big way. Blacklists are also known as blocklists.

Blacklists are used to combat spam in a very specific way. When spam is reported to one of the relevant spam fighting organizations the IP address the spam originated from is added to a banned or blacklisted IP addresslist. An IP address is the unique location of you or your website on the Internet - think of it as your "home address" online. To put it simply every "www.domain.com" Internet address has a matching IP address. Any email coming from your website domain also has a corresponding IP address. If your IP address is present on a blacklist then you're potentially wasting your time sending email to customers.

Why are you wasting your time? Modern spam blockers come with the most common blacklists installed and/or allow you to import updated blacklists into your spam blocker. This allows to you block a huge amount of spam but you may also, potentially, block legitimate email. Blacklists are not foolproof.

There are two types of IP address:

Dynamic - changes every time you connect to the Internet. Most commonly used for dialup Internet access. Spammers love these because they're very hard to track and 100% disposable.

Fixed/Static - All websites, most large companies and some individuals use fixed IP addresses. This can cause huge problems if they're reported for spamming.

When an IP address (dynamic or fixed ) is reported for sending spam it's added to a blacklist. There are three different types of blacklists:

Temporary An IP address placed on a temporary blacklist will have email coming from that IP address blocked for several hours. After a few hours the offending IP address is removed from the blacklist.