There are thousands of blocklists that are managed by various organizations and each one has different specifications for how a domain or IP ends up on their list. Sometimes known as a DNSBL or RBL these services are spam blocking lists for use by ISPs and mailbox providers.  Some lists are more important while others may not have a significant impact on your delivery rates. Below we have compiled a list of the blocklists that can most commonly be encountered.

High Impact Blocklists

Organizations listed as high-impact have the greatest impact on delivery of all of the blocklists.

• Spamhaus‍ is the most well-respected and widely used blocklist in the world. A listing at Spamhaus will have a negative effect on your ability to deliver emails to your customer’s inbox and can cause bounce rates of over 50%.  Evidence suggests that most of the top North American ISPs use Spamhaus to inform blocking decisions.

Medium Impact Blocklists

Organizations listed as medium-impact have the potential to impact email delivery.

• SpamCop‍ can have a significant impact on B2B email campaigns. However it is not widely used by major North American ISPs to inform blocking decisions.

Low Impact Blocklists

Organizations listed as low-impact are unlikely to impact email delivery.

• SORBS‍ uses several methods to identify potential spammers. Most of their lists use spam traps to identify problematic senders. But SORBS will also list a sender based on their own user complaints, if SORBS administrators have received spam from the sender, or if they identify other high-level sending behavior patterns characteristic of spammers.