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What is a Blacklist?

Tags: Email,  Security 

Generally speaking, a blacklist is any list of domains, IP addresses, or computers that communication is not allowed. Many kinds of blacklists are used in computing, and this guide is about blacklists from third parties that are maintained to prevent spam, bulk, and otherwise unwanted email messages.

Many third-party companies and organizations such as SORBS/Proofpoint, Barracuda, Spamhaus, RATS, and others maintain lists of IP addresses and/or domains of computers suspected of sending spam/bulk email.

Email providers will compare incoming email headers to one or more of these lists (or their own blacklists) and filter the messages to a junk or spam folder or reject the message altogether.

Email blacklists are very useful in preventing the spread of bulk/spam email, but sometimes they contain IP addresses that are not (or no longer) sending spam. Most of the major blacklist providers have a de-listing procedure, either by request or over time if they don't see any new spam reports.

The majority of blacklists require the service to be in an active state to have them removed. Therefore, if the IP is not currently active or assigned to a service, the removals tend not to go through.

Otherwise, the vast majority of listings are time-based. If the blacklist does not see any activity caused by the listing of the IP address for a certain amount of time, they will automatically de-list the IP.

If you find that emails sent from your hosting plan or server are not going to your recipient's inbox, it may be due to the IP address being on a blacklist. This is most commonly the case when setting up a new service, as the IP assigned to a new service was previously used on another service that might have been sending mail reported as spam.

While we cannot 100% guarantee the removal of an IP from a blacklist, as the blacklist is operated outside the scope of Hostwinds' control, we will happily assist with requesting that the IP be de-listed from any lists it's on and by making sure all the other email settings aren't contributing to the problem.

Written by Hostwinds Team  /  June 5, 2021