How Email Blacklists Work, and Tips for Avoiding the Spam Filter

Spam filters make use of a variety of methods to help keep unwanted messages out of our inboxes. However, while the technology is constantly evolving to better keep up with increasingly aggressive spam campaigns, the underlying technology used to block junk mail remains much the same. A major component of any spam filter is the email blacklist. These are lists of IP addresses and domain names that have already been flagged as spam by major mail providers. Many of these blacklists are publicly available, allowing businesses and mail and internet service providers to use them for deciding whether to reject emails from specific address ranges.


Blacklists identify spammers based on the domains or IP addresses they send mail from. However, to increase their effectiveness, they may also include IP address ranges to block a wider range of offending addresses. For example, a blacklist might include all email addresses sent from a specific region, internet service provider or even an entire country. Internet service providers and email providers use these blacklists as references for blocking both spam and potentially malicious emails. From a marketer's perspective, most email marketing platforms monitor the major blacklist resources to see if any of their customers end up on them.


In discussions about email blacklisting, you may also encounter the term greylisting. Similar to how a blacklist works, a greylist includes a list of email addresses, domain names and IP address ranges. However, rather than being a list of spammers, these databases include listings of bulk senders, which includes everyone who sends promotional emails. Greylists are useful for keeping inboxes of private domains clean in which owners do not want to receive marketing emails of any kind, regardless of whether they signed up or not. While it's not likely that you can get removed from a greylist, it should not affect deliverability or the performance of your campaign.




How Do Blacklists Effect Email Deliverability?


Not all blacklists are made equal. After all, anyone can set up a blacklist for their own email server or account. For example, Gmail, which is the world's most popular free email provider, allows users to block senders by adding them to a blacklist. People can easily share their own blacklists with the public, and while smaller lists rarely have a discernable effect on larger internet and mail service providers, there are much larger ones created by companies, such as service providers themselves. Most email providers also allow their users to report spam and phishing emails for review, in which case they may end up on the provider's blacklist.


Unsurprisingly, larger blacklists, such as those belonging to major service providers like Microsoft and Gmail, carry a lot more weight, and that translates into a higher bounce rate for anyone on the list. Other major blacklist resources, which are also available to the public, include the Composite Blocking List by Spamhaus, Spamcop, Invaluement, Barracuda and Sender Score. If your address ends up on any of these major lists, then deliverability will suffer greatly. Fortunately, most email marketing platforms will try to get their servers removed from these lists, but only if there is a case in favor of removal.


Bigger internet service providers often have their own internal blacklists, as do major companies that maintain their own private mail servers. Outsiders cannot access these blacklists, but they don't tend to have a major impact on deliverability anyway. Oftentimes, they work more like greylists as well, in that companies use them to prevent being bombarded with sales emails. Since there's no point in sending sales emails to recipients who aren't interested in them anyway, this should not present a problem for your marketing campaign. However, when it comes to public blacklists, you should take a proactive approach to avoid appearing on them in the first place.




How Can You Get off an Email Blacklist?


Blacklists are useful. In fact, they're one of the most important weapons in the fight against spam. However, they are also generated by machine intelligence for the most part, and that means they can sometimes confuse legitimate email senders with spammers. If you find yourself on a blacklist, despite not sending any spam emails, then you'll often be removed automatically after some time has passed in which the address has not been associated with any spam attempts. There are various resources which you can use to check if you've been added to a public blacklist, including the blacklist curators themselves.


Most senders who end up on email blacklists do so because their emails have been flagged as spam too many times. Monitoring your online reputation is essential to maximizing email deliverability, and that means keeping a close eye on the public blacklists to ensure that your address doesn't appear on any of them. If, however, you do discover you're on one of these lists, it's crucial that you take immediate action, not least because it may take some time to get off the list. You can quickly find out if you're on any of the major blacklists by visiting the websites of any of the previously mentioned public blacklist providers.


Waiting to be automatically removed from a blacklist is a last resort, since it can take many months, during which your email deliverability will suffer even if you're sending legitimate content to the right people. While light offenders should get removed in less time, taking steps to remove your IP address yourself is almost always preferable. Fortunately, most public blacklist providers offer a way for people to remove themselves, provided you explain your case clearly and can convince them that you are innocent of any wrongdoing. Nonetheless, if your address gets listed again, you're unlikely to get another chance.




How to Avoid Email Blacklists in the Future




Many people will tell you that the best way to avoid landing on the public blacklists is to avoid sending any spam in the first place, but the reality is a bit more complicated. Constantly improving deliverability should be considered a key goal of any email marketing campaign. After all, there's absolutely no point in sending emails to people who aren't interested in receiving them, so you should always take a proactive approach towards keeping your mailing list clean. Doing so won't just keep you away from the blacklists - it will also greatly enhance the effectiveness of your marketing strategy by increasing open and response rates.


Most importantly, a robust email marketing campaign starts with a great mailing list that consists of engaged subscribers who actually look forward to hearing from you. Remember, it's also important to prioritize quality over quantity, which is something that companies that buy mailing lists don't seem to understand. To begin with, you should familiarize yourself with the law, such as the CAN-SPAM legislation in the US. Many countries have similar legislation, which makes it illegal to use deceptive advertising methods, mask your real email address and, more importantly, send promotional content to anyone who has not given you their express permission.


Unfortunately, many email marketers aren't clear on precisely what constitutes permission to send promotional content. For example, a lot of businesses falsely assume that receiving an email during an online transaction grants permission. In other cases, businesses might include a checkbox that has already been ticked during the checkout process, in which case many people don't bother to deselect the box even if they aren't interested in receiving promotional emails. To keep on the right side of the law and maintain the integrity and quality of your mailing list, you should instead use a double opt-in process.


With a double opt-in process, a user will subscribe to your mailing list using a signup form on your website or by ticking a relevant box while checking out in the case of online shopping. The user will then receive an email with a link asking them to confirm their subscription. Only once they've clicked on the link will they be added to your mailing list. This ensures that there are no misunderstandings during the process and that the individual is genuinely interested in receiving promotional content from you. However, you should still include a working unsubscribe link with every email, since it is a legal requirement to do so.


Last, and certainly not least, is the importance of producing quality content with actionable information and personalized promotions that are most likely to be of genuine interest to the recipient. You should also use mobile-friendly templates with clean coding and clear formatting. Always remember to craft useful and engaging subject lines, and avoid the common tactics that spammers use, such as excessive use of capital letters and exclamation marks. Finally, you should consider your email campaign as a long-term process of continuous improvement whereby you actively maintain your mailing lists and be consistent with your sending frequency.






Email marketing offers the greatest return on investment of any form of marketing, but that doesn't mean it's always easy to get right. To avoid the spam filter and maximize deliverability and engagement, you'll need to work hard to craft content that your recipients look forward to reading. Ultimately, avoiding the blacklists is all about sending useful emails to an engaged audience, and there are no shortcuts to that process.