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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.

Permanent

When an IP address is added to a permanent blacklist any email server configured to block email from this list will never receive email from that range of IP addresses again.

Comprehensive

This is the most damaging of blacklists. Not only does it block a single IP address it also blocks the IP addresses next to it. For example if the IP address 192.156.66.67 was added to a comprehensive blacklist then all IP addresses close to 192.156.66.67 will also be blocked. This can be a huge problem for those website owners using virtual hosting because if your host has ever appeared on a blacklist then you're also on the same blacklist, by default, because of the shared hosting from the same IP range. It's important for all website owners to check whether or not they're on a blacklist. You'll need your IP address (available from your webhost) and you can check your blacklist status at: www.mail-abuse.org/cgi-bin/lookup

Blacklists are a necessary evil due to the volume of spam being sent each day but are not an exact science. Take a few moments from your day and ensure that your website or email address is not being blocked.