Deliverability Indicators: Understanding Email Blocklists and What They Mean For Customer Engagement

With over an estimated 300 billion messages sent daily in 2023, email continues to be an integral tool in brands’ marketing arsenal. Recent figures indicate that email’s ROI can be up to 4,200% when this channel is leveraged correctly. Since its efficacy relies largely on a sender’s ability to reach the inbox, adherence to deliverability best practices remains the foundation of any impactful email marketing program.As the industry continues to evolve, your ability to measure success is dependent on your ability to analyze and interpret your sending results. But what metrics matter, and which aren’t as important? What should you monitor to ensure you’re maintaining high deliverability, and how should you react when faced with unexpected or unpleasant outcomes? Join us as we take a deeper look at each of the signals that impact your deliverability. This is Deliverability Indicators, our ongoing blog series that provides a comprehensive breakdown of the factors that influence your email sender reputation.

This edition’s focus: Blocklists

Alternate terms: Blacklist, blocking list, denylist, disallowlist

What they are: Blocklists are lists of IPs, IP ranges, or domains that are associated with spam. Contrary to their name, blocklists do not actually block mail; they are merely databases that mailbox providers can reference to aid their delivery decisions.

Historically, blocklists had a much larger influence on deliverability, but as spam filtering has evolved, their importance has diminished. In the 1990s and early 2000s, a blocklist would be the first place to check if mail was being rejected. But once consumer mailbox providers became better able to assess sender reputation, they augmented blocklists with algorithms which integrate internal historical delivery data to drive more accurate delivery decisions. As a result, it is entirely possible to be blocklisted these days and see minimal impact on your traffic, depending on the blocklist and recipient domains.

Who they are: Because there are no restrictions or governing bodies that regulate blocklists, anyone with the inclination can establish one. There are over a hundred known email blocklists of known significance, and innumerable others published (though utilization may be negligible).

The most impactful blocklist is Spamhaus, whose listings can reduce deliveries by upwards of 60%, depending on list composition. Other common blocklists include SORBS, UCEProtect, SpamRats, Invaluement, Abusix, Proofpoint, Spamcop, SURBL, and 0Spam. Consumer mailbox providers like Microsoft can also maintain their own blocklists.

Why they happen: Blocklistings are most often due to spam trap hits, spam complaints, and other signs that an IP or domain is exhibiting signs of sending unwanted or malicious messages.

Where to see them: Several online tools offer blocklist checks for a domain or IP blocklisting. Popular options include MXToolbox and Wombatmail. Keep in mind, a deliverability problem may not be due to a listing. More likely, a listing and a deliverability problem have the same source cause (e.g. spam trap hits).

When to worry: If your IP or domain is on a blocklist, first assess the impact. Is your mail being blocked (i.e. rejected)? Do the bounce codes or reasons specify a blocklist as the root cause for the rejections? Braze customers can review their bounce reasons in the Message Activity Log, or by leveraging Currents. If the full bounce messages do not refer to a blocklist, then it is unlikely that the listing is causing the rejections. However, being listed still indicates that your IP or domain is potentially exhibiting signs of sending spam.

Even if mail isn’t being rejected, you should still review your practices to ensure that your opt-in process is working as expected, your sunset policy is active, and you’re targeting sends to your most-engaged audience members. A blocklisting can portend future deliverability problems, so treat their occurrence as a warning even if there aren’t yet noticeable consequences. Some more aggressives blocklists may list an entire range of IP addresses or even an entire Email Service Provider (ESP). These broad blocks are unlikely to affect your deliverability due to their low adoption.

Prevention: A strong opt-in process and sunset policy are the primary defenses against being blocklisted. Implementing confirmed opt-in ensures that all addresses on your mailing list belong to legitimate subscribers. A sunset policy eliminates the risk of sending to unengaged addresses that have been repurposed as spam traps. Braze Deliverability Services customers receive customized guidance to avoid blocklistings, and tailored recommendations for remediation should listings occur.

Mitigation: Once listed, you’ll need to review your recent sends to determine the potential root cause(s). A listing may be due to changes in your audience (such as updated segments that changed your user targeting), a new lead generation source, or an attempt to reactivate lapsed users. If you collect addresses with an online signup form, it should be secured with a captcha and/or honeypot field to deter form abuse that contributes to blocklistings. Some blocklists provide removal instructions on their sites, while others will automatically delist after an unspecified period of time has passed since seeing problematic sending behavior. If requesting removal, you will need to include details about how subsequent listings will be prevented, such as the steps you’ve taken to improve the opt-in process, or updates to your sunset policy. Additionally, do not pay to be removed from a blocklist—impactful blocklists do not require payment to process delistings.

Key Takeaway

While the significance of blocklists has decreased over time, it’s still important to assess the impact and review practices to ensure emails are reaching the intended audience. Want to learn more on how to strengthen your email sending reputation? Check out our guide Demystifying Email Deliverability.

Be on the lookout for the next edition of Deliverability Indicators, which is all about spam traps!