Blocklists and blocklisting

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What is a blocklist?

An email blocklist is a real-time database, usually of an organization of the anti-spam community, that uses criteria to determine if an IP address or a domain is sending email that is considered to be spam and must therefore be prevented from further malicious activities. It helps the business using it to decide if emails received should be delivered to the final recipient or if they should rather be blocked because the sender is not trustworthy.

Public blocklists

When speaking of blocklists, a blocklist mostly refers to public blocklists that any business can include into their set of anti-spam measures. In this context, these are often called Domain Name System-based Blackhole Lists (DNSBL) or Real-time Blackhole Lists (RBL), simply because they are operated and updated in real-time.

Blocklists are used by organizations like Internet Service Providers (ISPs), Free Mailbox Providers (MBPs) and anti-spam vendors but also private businesses as well as institutions of the public or educational sector to keep spam away from their infrastructure and their user’s mailboxes.

ISP-internal blocklists

Since often heavily targeted by spammers, Internet Service Providers also have their own internal blocklists that a sender can get listed on when spammy behaviour is noticed. Any potential blocklisting in this regard only impacts sending to the corresponding ISP however.

Optimizely Campaign blocklist

Not a blocklist in that sense is the internal Optimizely Campaign blocklist. This is a so-called suppression list where you can manage recipients who no longer want to receive messages from you.

Types of blocklistings

Blocklistings are generally applied to the sending IP or the sender domain of a potentially untrustworthy sender.

IP-based blocklisting

An IP blocklisting applies to the IP address used for sending - the unique number given to every computer, server or other device when connecting to a network. If potential spam emails keep coming from the same IP address, it will then be added to a blocklist as an indication of junk. In most cases this means that further emails from the same IP address will also be blocked. Sometimes a spam issue can be so severe that an entire IP range is blocklisted.

Domain-based blocklisting

A domain blocklisting applies to the sender domain, no matter which IP addresses the potential spam emails are sent from. If a lot of spam originating from a certain domain is detected, this domain will be blocked. Even if you switch to a different IP, if the domain is still blocked, your emails will not be delivered.

How do I get blocked?

Blocklists are kept up to date usually in real-time with data provided by ISPs. An IP or a domain is quickly blocked if repeated signs of spamming behavior are noticed. However, even reputable senders can also be blocked, if one or more of the following problems have occurred:

  • Sending to spam traps. Spam trap email addresses do not belong to any real person, they are honey pots set up by the Internet Service Providers or blocklist operators to identify senders that do not respect opt-in or do not practice list hygiene. These could have been harvested from websites or alternatively, these might have been old, inactive email accounts that were converted into spam traps after a too long time of inactivity.
  • High user complaints. Spam complaints are reports ISPs receive when your recipients click the spam button in your emails. These make ISPs assume that the emails you send are unwanted, be it due to a lack of opt-in, too high email frequency, insufficient relevance of the email content or a lack of list hygiene. The more complaints, the more likely the responsible sending domain or IP address will be blocked.
  • Sending to a high number of inactive email accounts. Sending to inactive email addresses often leads to high hard bounce rates. This is an indicator for an ISP that a big part of your recipient list is either very old or has not been acquired using double opt-in.
  • Big surge in recipient list size. A good email marketing list will grow sustainably over time as new customers or leads sign up to receive your emails. When a list grows disproportionately in a short period of time, ISPs might suspect that it has been purchased or crawled. Furthermore, big spikes in sending volume are a typical sign of spammers and are therefore often distrusted.

If your sending activities are in line with the Optimizely Campaign Deliverability best practices, no blocklistings are expected.

Important blocklists

Nowadays, there are countless blocklists which are used by the anti-spam industry, Internet Service Providers, and major businesses around the world. However, some of them have a much greater impact on deliverability than others. Optimizely and Optimizely's customers must ensure collaboratively that the sending IPs and domains are not listed, otherwise this can have a very negative impact on your deliverability. The most important blocklists are listed in the table below.

Blocklist Description Impact on deliverability
Spamhaus Spamhaus runs numerous lists that are used by companies and ISPs around the world – such as Spamhaus Block List (SBL), Domain Block List (DBL), and Policy Block List (PBL). SBL is the one that most senders are likely to run into through the sending of marketing emails to ISPs. Spamhaus includes sending domains and IPs of known spam sources as well as spam trap addresses commonly found in purchased lists into their detection mechanisms.


Affects delivery to ISPs, anti-spam appliances and B2B addresses

Invaluement The Invaluement blocklist consists of the ivmURI list which lists URLs that have been identified as malicious, the ivmSIP list that is based on IP addresses from known botnets and snowshoe spammers as well as the ivmSIP/24 list that lists entire IP ranges or sub-nets where spamming activities have been detected. Invaluement mostly relies on spam complaint data provided by web hosts and mailbox providers to generate blocklistings which are used by a variety of reputation systems and anti-spam appliances.


Affects delivery to ISPs, anti-spam appliances and B2B addresses mostly in the UK, US, and Nordic countries

SpamCop The SpamCop Blocking List (SCBL) lists IP addresses that have transmitted reported spam to SpamCop users. With the SpamCop Domain Blocking List (SCdBL) SpamCop have also started to offer domain-based blocklisting, which occurs based on the sender domain and signature used. SpamCop is mostly known as an anti-spam service one can report spam to, but they also analyse email sent to spam traps. Furthermore, SpamCop applies a scoring system to weigh reported email before an IP or domain is listed.


Affects delivery to B2B addresses

SORBS Spam and Open Relay Blocking System (SORBS) is a blocklist that mainly works with spam traps and a hit of one of their traps leads to the blocking of single IP addresses or entire IP ranges.


Affects delivery to B2B addresses

Barracuda The Barracuda Reputation Block List (BRBL) is a service operated by the security and anti-spam vendor Barracuda Networks. It mainly lists IP addresses that have transmitted email to spam traps. If an IP address passes some basic tests, such as for proper server configuration, the email is then scored against the Barracuda Reputation System which takes into account any potential threats detected by the Barracudas anti-spam appliances and if considered spam, immediately added to the BRBL.


Affects delivery to B2B addresses

NiX Spam NiX Spam is a German blocklist that also works with spam traps and blocks the IP address used for sending the email that hit their traps.


Affects delivery to B2B addresses mostly in the DACH region

UCEPROTECT UCEPROTECT is a spam protection database founded in Germany which is mostly used by their members in Germany, Austria and Switzerland. It mostly affects domains of public bodies and private businesses and is based on spam trap hits and spam complaints.Listings may occur to single IP addresses (level 1), multiple IP addresses (level 2) and finally all IPs of entire IP networks (level 3).


Affects delivery to public bodies and B2B addresses mostly in the DACH region

Optimizely proactively monitors hundreds of known blocklists, keeps you updated on blocklistings that could potentially impact your sending and provide strategy consulting on how to avoid them.