There are many ways to fight spam, and mailbox providers such as Gmail and Microsoft have developed complex, proprietary behavioral algorithms to keep their customers free of spam and phishing. Most mailbox providers consider both the positive metrics (open ratio, click ratio) as well as the negative (spam complaints, moving email to the spam folder, unsubscribe ratios) metrics on top of their own antispam solutions when determining IP/domain reputation.  

However, to evaluate if the campaign should be accepted by the server, some mailbox providers rely on an external opinion. There are vendors that keep real-time data on the IP addresses and domains known for spam-like activities, like phishing or disregarding email best practices. These vendors are called remote block lists or real-time blackhole lists (RBLs).

RBLs: What they are and why they’re needed

RBLs are a collection of IP addresses or domains identified as sources of spam or abusive email practices. They are updated in real-time, allowing for immediate action against new threats. Since RBLs are DNS-based, mailbox providers and businesses may perform a DNS lookup by quickly checking whether the originating IP address or domain is listed on an RBL. If the IP/domain is listed, the email will bounce or will be sent to the spam folder. 

RBLs are a cybersecurity solution designed to protect mailboxes from spam and phishing. Everyone can employ an RBL on an email infrastructure as a safety measure, as RBL lists can be a useful tool in the fight against spam and other types of email abuse. They are not perfect and can sometimes cause legitimate emails to be blocked or marked as spam. Therefore, it’s important to use RBL lists in combination with other spam filtering techniques, and to periodically review blocked messages to ensure that legitimate emails are not captured by the filter.  

There are hundreds of RBL vendors available, some of them have a significant impact on email delivery, while others have little to no impact on the IP/domain. Seeing your IP address or domain listed does not necessarily indicate deliverability issues such as spam placement or increased bounce ratios. It all depends on the vendor that listed the IP – there are vendors that are not used widely by mailbox providers and businesses – in this case, such vendor has no power to get your IP blocked or inbox placement altered. On the other hand, there are many well-respected vendors whose listing can even prevent you from sending further communications.

What could get IP/domain listed by an RBL?

1. Spam traps are used both by mailbox providers and RBLs to evaluate a senders’ list quality. Hitting spam traps has a major influence on the RBL decisions to list an IP/domain. Senders should always aim to have zero spam traps in their lists by implementing double opt-in, actively removing old contacts from the lists, and removing all invalid addresses.

 

2. High spam complaint rates are known for getting senders blocked. High spam complaint ratio or a direct spam complaint (subscriber sent a spam complaint directly to the RBL) can cause a drop in the IP/domain reputation, increase spam placement or getting the IP/domain listed and blocked. The prominent way of reducing spam complaints is sending the campaign to subscribers that gave explicit permission to be contacted and still remember your brand.

 

3. Rapid volume spikes can get an IP/domain in trouble too. Mailbox providers and RBLs expect a stable email volume to be sent, as wrongdoers often abuse email channel by sending malicious campaigns as quickly and as many as possible. Rapid volume increases are often treated as malicious both by mailbox providers and RBLs, possibly getting sender blocked to prevent any damage.

 

Key RBLs that matter for your deliverability

1. Spamhaus: This is one of the most impactful and recognized blocklist. It’s highly effective for detecting and preventing spam-like behaviors, including spam and phishing. Being blocked at Spamhaus could prevent an IP/domain from sending entirely, as it can easily block half of the total email volume. If you see your IP/domain listed on Spamhaus, it usually indicates deeper issues with best email practices which need to be addressed as soon as possible, such as spam trap hits, high complaint ratios, sending to inactive audience, purchased lists, or a combination of these factors.

 

2. SpamCop: They have been around since 1998 and are actively fighting against spam and phishing today. Since SpamCop is efficient and free, many businesses have decided to implement SpamCops’ SCBL to protect their email infrastructure. A listing could potentially impact the high number of subscribers, but it is not as severe as Spamhaus. SpamCop delists IP automatically after 24 hours after the last spam attempt, however they do not hesitate to block the IP again if the good email practices have been omitted. They do calculate the “reputation points” for the IP and are transparent about how they calculate those points.

 

3. Barracuda Networks: They developed the Barracuda Real-Time Blocklist (BRBL) which protects their users from spam and malicious actions. Their automated spam trap and email rating systems add IP addresses to the blocklist if a sender is qualified as a bad actor by those systems. They also scan the IP for being a botnet node or being an open proxy, but most senders get listed because of direct spam complaints or triggering other automatic detection systems including spam trap hits. The listing is not as significant as Spamhaus or even SpamCop, but it still can affect a lot of B2B customers. 

 

4. Cloudmark and Proofpoint: Cloudmark operates Cloudmark Sender Intelligence (CSI), which is a blocklist working like the ones mentioned above – the IP reputation is determined by data from various sources, including direct user reports and spam traps. Getting listed by Cloudmark can be treated as severe, as it impacts multiple domains. Proofpoint, which acquired Cloudmark, also owns its own RBL which is qualified as high-impact RBL.

 

Getting blocked by RBLs can cause deliverability and inboxing issues, and a listing should be addressed as quickly as possible. But not every RBL is equal, and some are more significant than others. The conclusion for senders is simple – sending to an engaged, organic audience is highly recommended to avoid any blocks or issues with RBLs. Following the best email practices is always encouraged and the RBLs are no exception.

To get more tips about email deliverability, download our one-pager guide, How to maximize deliverability and steer clear of the spam folder.