If you're in email marketing, deliverability is everything. You can have great content, clever subject lines, and an enticing offer, but if your emails land in the spam folder or don’t get delivered at all - none of it matters.
Things like authentication protocols (SPF, DKIM, DMARC), engagement rates, and consistent sending habits all impact your sender reputation. When there are issues with even one part of those elements, it can weaken your sender reputation and hurt your deliverability, but one of the most serious consequences of it is getting on the so-called “Public spam (block) list”. Let’s break down what these are, how they affect your campaigns, and what you can do to avoid getting listed.
What Are Public Spam Lists?
Public spam lists, also known as DNS-based Block List (DNSBLs) or Real-time Block Lists (RBLs), are databases of IP addresses or domains that have been flagged for sending spam or malicious activity. They are publicly accessible and used by mailbox providers, spam filters in private mail servers, and email security services to help them decide which emails to let through and which to block or send to the spam folder.
How Do They Affect Your Emails?
So, you have sent emails to your contacts, and in the analytics, you see a delivery bounce, for example:
smtp;550 An URL in this email ( [URL] ) is listed by Spamhaus DBL. See https://check.spamhaus.org/
Or:
554 5.7.1 Service unavailable; Client host [IP] blocked using b.barracudacentral.org; http://www.barracudanetworks.com/reputation/?pr=1&ip=IP
The first bounce says that the email contains a link that has been listed by Spamhaus (it’s a public spam list). The second one says that the sending IP is blocked using Barracuda Networks (another public spam list). In both cases, the email was blocked and not delivered. And at this point, you might think that your emails are blocked because the public spam list is blocking you. But that's not quite true... It’s important to understand that public spam list providers don’t block your emails directly. Instead, mailbox providers (like Gmail, Yahoo, Outlook, etc.) or any other private mail server use these lists as part of their filtering systems. Being on one of the public spam lists doesn’t automatically mean all your emails will be blocked, but it does increase the risk of it happening significantly. However, not all spam lists are equally valuable for anti-spam mail servers. Some are more trusted (the top 3 are listed below), and hitting their lists has a greater negative “weight”, while others are evaluated as “informative”.
But in any case, in spite of the fact that the recipient's mail server makes the decision to block the emails - if the public blocklist is specified in the delivery error as the reason for blocking, then you should also contact this blocklist to solve the problem (for delisting).
What Triggers a Listing?
Getting listed isn't random. Here are the usual suspects:
-
Spam Traps: These are email addresses designed to catch spammers. There are two main types:
- Pristine traps are brand-new addresses, never used for sign-ups. Hitting one suggests shady list-building tactics.
- Recycled traps are old inboxes that used to work but have been dormant for years. Sending here shows poor list hygiene. - Unprotected Forms: Forms that don’t have CAPTCHA or other bot protections can be exploited to submit fake or malicious email addresses. Unprotected forms also lead to Email List Bombing attacks.
- User Complaints: High complaint rates from recipients marking your emails as spam can trigger a listing.
- Suspicious Sending Patterns: Unusual spikes in sending volume or frequency can indicate a compromised account or poor list hygiene.
Types of Datasets / Listings
Public spam lists don’t just randomly add email senders to their dataset. They use specific types of listings based on what they detect as suspicious or spammy behaviour. Here are the main types you should know about:
1. IP-Based listing
What it is:
A dataset of IP addresses which were seen sending spam or acting suspiciously. If your sending IP is listed, there is a high chance that none of your emails will get through from that IP for some recipient servers, no matter how good your content is.
Why it happens:
- The IPs are sending high volumes of unsolicited email.
- The server is misconfigured (like being an open relay).
- The IPs belong to a sender that allows abuse.
2. Domain-Based Listing
What it is:
Instead of listing IPs, the spam list targets a specific domain name. This could be:
- The sending domain (e.g., email.yourcompany.com)
- A domain in your links (e.g., URLs in the body of your email)
- Your SMTP domain (the technical domain, return-path domain)
Why it happens:
- Your domain was reported for sending spam.
- The links in your email point to a site that’s been flagged as malicious.
- The domain was used in phishing or malicious content.
Top 3 Most Influential Public Spam Lists
-
Spamhaus
Probably the most trusted and widely used. Spamhaus tracks IPs and domains involved in spam or malicious activity. Their datasets, like the SBL (Spamhaus Block List) and DBL (Domain Block List) etc., are used by mailbox providers and some email service providers worldwide. -
SpamCop
SpamCop relies on user reports to list IPs and domains flagged as spam. -
Barracuda
Barracuda Networks offers a reputation-based service that tracks IP addresses and domains engaged in sending unsolicited emails. Their Barracuda Reputation Block List (BRBL) is used by many mail servers and email security systems to help filter out harmful content and prevent spam.
How to Prevent Getting Listed on Public Spam Lists
Here’s what you can do to stay off these lists, and stay in the inbox:
- Use Double Opt-In: Verify the mailboxes of new subscribers to ensure only real people join your contact list.
- Clean Your List Regularly: Frequently clean your email list by removing inactive or unengaged contacts.
- Authenticate Your Emails: Implement SPF, DKIM, and DMARC to verify your sending domain and protect against spoofing.
- Secure Your Sign-Up Forms: Use CAPTCHA and other anti-bot measures to prevent fake or malicious sign-ups.
- Monitor Your Reputation: Use tools to track your sender score and check if your IP/domain is on any blocklists.
- Follow Industry Best Practices: As always, be safe. Avoid spammy content, maintain consistent sending patterns, and don’t blast emails to everyone all at once.
Final Thought
Public spam lists aren’t out there to get you - they exist to protect email users. The idea of blocklists came from a desire to reduce the amount of spam and unsolicited content for users, and like any other protector - they believe very strongly in their mission. In most cases, all you have to do for delisting is to fill out a form on a particular spam list, and that sounds simple enough.... But if you land on such a list, it’s a sign that something in your email practices needs fixing. Before requesting a delisting, you need to:
- Analyse the reasons for the listing
- and take steps to fix the reasons and prevent them from happening again.
If you skip these steps and proceed only with a blocklist removal request, in case of a next listing occurs the requirements for delisting may be tightened and in the extreme case there may be no delisting at all, resulting in poor deliverability for a longer period of time.
Hopefully, with an understanding of how these lists work and following deliverability best practices, you’ll keep your reputation strong and your emails where they belong: the inbox.