Ask a Deliverability Expert: What’s a Blocklist, and Why Should I Care About Them?

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I’ve read stories about email blocklists, but I’m not sure what they are or who runs them. Is this something I should be worried about with my newsletter? Could they stop me from sending my newsletter to readers?

Let’s start with a straightforward answer: A blocklist is a list of IP addresses or domain names that are identified as sources of spam or other types of unwanted emails. These blocklists are maintained by big international organizations — more on them in a moment — and internet service providers (ISPs) use blocklists to block or filter out emails from these sources in an effort to protect their users from spam.

You probably have a sense of what spam is already, but in the deliverability industry, we’d officially define it as unsolicited bulk email that is sent for the purpose of advertising or promoting a product or service. Spam is often sent in large quantities and can be annoying and disruptive for users. As spammers have ramped up their efforts over the past few decades, ISPs and other organizations began to develop ways to filter out spam and protect their users from receiving it. One way they did this was by creating blocklists of known spam sources.

Who runs these blocklists, and how do they work?

There are many different blocklists out there, each with their own criteria for determining which IP addresses and domains to include. Four of the most popular blocklists are:

• Spamhaus — This is a non-profit organization that maintains several blocklists of known spam sources.
• Spamcop — This is another non-profit organization that maintains a blocklist of known spam sources..
• Barracuda Central — This is a commercial blocklist maintained by Barracuda Networks, a provider of security and data protection products.
• Google Safe Browsing — This is a blocklist maintained by Google that is used to protect users from phishing attacks and malware.

Each of these blocklists is widely trusted by ISPs around the world. Each operates slightly differently and has its own criteria for determining which IP addresses and domains to include. But they all have one thing in common: They are used by inboxes to block or filter out spam in an effort to protect users.

Following best practices allows you to reduce issues with blocklists

Inbox providers like Gmail or Yahoo monitor your reputation over time. If you’re a good sender — readers tend to open and engage with your newsletters, and you follow email best practices — you’ll probably have a pretty good reputation among the inboxes. A good sender reputation is essential for ensuring that your emails are delivered to your recipients. But things can change pretty quickly if you end up on a blocklist. (Before we go any further: Here’s a free tool from MX Toolbox to check to see if you’re on a blocklist.)

If you end up on a blocklist, you may find yourself unable to deliver your emails to your readers. The blocklist effectively tells the inbox: This sender seems shady — send their emails to spam.

The good news: There are a number of best practices you can follow to reduce the chances of your IP address or domain being placed on a blocklist and negatively impacting your email deliverability.

When I work with any organization that sends emails, I always recommend a few simple steps::

• You should always obtain explicit consent from recipients before sending them any emails.
• You should provide a clear and easy way for recipients to unsubscribe from your emails.
• You should regularly clean your email list to remove inactive or invalid email addresses.
• You should only send relevant and valuable content to your recipients.
• You should use a reputable email service provider (ESP) that follows best practices and has a good reputation.

These are all fairly simple steps — there’s nothing here that should come as a huge surprise. But just by following these best practices, you can help to ensure that your emails are delivered to your recipients’ inboxes and that your reputation as an email sender remains intact. Senders who follow these practices generally don’t have issues with blocklists.

Why being listed on a blocklist affects your sender reputation and deliverability

What might cause your email to land on a blocklist? In addition to failing to follow those five simple steps above, a few other things might get you in trouble with a blocklist.

Acting like a spammer

Spammers tend to behave in predictable ways. They use deceptive subject lines to trick readers into opening. They email readers who didn’t grant permission to be emailed. They purchase email lists. They send to readers who’ve previously marked their emails as spam. They use hidden or invisible text in their emails. They send email with weird attachments, like .exe, .vbs, and .bat files, which are sometimes used to deliver malware to readers. They try to spoof legitimate domains. They try to steal personal information from readers.

Utilizing any of these tactics — and definitely doing several of them — might catch the attention of a blocklist.

Failing to obey local privacy laws

If you ignore laws like CAN-SPAM, in the United States, or CASL, in Canada, you may end up on a blocklist. The companies behind blocklists expect you to follow your local laws, and failing to do so will have consequences for your newsletter.

Sending emails to spam traps

Spam traps are email addresses that are specifically set up to catch spam. These are email addresses that blocklists know aren’t real — so if they end up on your list, it’s a signal that you might be engaging in shady practices, like buying emails. (How else could they have ended up on your list? They’re not email addresses that belong to real people!) Sending emails to spam traps can result in your IP address or domain being placed on a blocklist.

Using a fake or outdated “from address”

These can be a red flag for spam filters. Make sure the email address you send from is up to date, and make sure that the email address you send from also has an inbox that can receive emails.

Failing to authenticate your emails

If you haven’t set up SPF or DKIM for your sending domain, the inboxes may not be able to prove that you are who you say you are, and that could cause issues with a blocklist.

Suddenly adjusting your sending cadence

Let’s say you normally send a million emails per day, and consistently do so for a few months. Then one day, you send 50 million emails, and then 100 million the next day. A blocklist might get suspicious — what’s behind that rapid change in sending patterns? — and add you to a blocklist.

What happens if you do end up on a blocklist?

If your IP address or domain is placed on a blocklist, it can significantly impact the deliverability of your emails. Emails from blocklisted IP addresses and domains are more likely to be blocked or filtered by ISPs and other organizations, which means that your emails may not reach your intended recipients. Your emails may land in the spam folder — or not even make it there. You may not get as much as a bounce message sent back to you. This can lead to decreased engagement with your emails and a negative impact on your business or email strategy.

In addition to affecting your email deliverability, being listed on a blocklist can also have a negative impact on your sender reputation. If you have a poor sender reputation or are listed on a blocklist, it can be difficult to recover and may take weeks or months to rebuild your reputation. While you’re doing so, you may struggle to get your emails delivered to readers. This is why it’s so important for email senders to follow best practices and avoid being placed on a blocklist in the first place.

Blocklists are important — but they’re not anything to fear

Blocklists absolutely matter. They play a critical role in the deliverability of your emails. If your IP address or domain is placed on a blocklist, or if you have a poor sender reputation, it can be difficult to get your emails delivered.

The good news: These blocklists are maintained by reputable orgs that are truly looking for spammers and scammers, not businesses or individuals who are just trying to talk with their audience. By following a handful of common-sense practices and maintaining a good reputation, you shouldn’t have an issue with blocklists. You’ll be able to stay in the good graces of the blocklists — and keep delivering emails to your audience.

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