Understanding email blocklists

90% of all emails sent worldwide each day are considered spam emails. Without blocklists and spam filters, it would be nearly impossible for anyone to read anything in their inboxes.

Email Blocklist

A blocklist is a list of IP addresses or domains blocked for several reasons. These lists are kept in real time and are available to the public. Receiving mail servers will query a blocklist to gather information on the reputation of an IP address or domain of an incoming email before deciding whether or not it will deliver the message. Ending up on a blocklist doesn't necessarily mean the IP or domain is a spammer. It just means that the IP has been reported as a known source of spam.

An excellent online tool for blocklist checks is MX Toolbox.

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Email blocklist controllers

Blocklist third-party databases exist that email providers can query. The receiving mail server only uses the information to aid in its decision on whether to refuse an email or not. The blocklist service does not determine if the email is accepted or rejected by the receiving mail provider. 

Surriel (PSBL)

An easy-on, easy-off blocklist that does not rely on testing should reduce false positives because users can remove their ISP's mail server from the list.


The Spamhaus project is one of the most reputable blocklisting companies providing real-time anti-spam protection for internet networks worldwide.


The SpamCop Blocking List (SCBL) lists IP addresses that had mail reported as spam by SpamCop users.

How blocklists work

When a mail server receives an email, the mail server will reach out to the third-party blocklist database to find out if the IP/domain has been listed as a known source of spam. The blocklist will return with a yes or no reply: the IP/domain is not listed as a source of spam in the blocklist database for the provider being checked against.


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Public blocklist

Public blocklists are like the ones mentioned above. Public lists are shared via the internet and used by anyone to check IP addresses and domains, such as Spamhaus, Spamcop, and Surriel.

Private blocklists

Companies have created private blocklists for their security and spam filtering purposes and are not made publicly available. The methods used by the company to determine if an IP or domain is listed on their private blocklist are proprietary information. If you are blocked by one of these lists, you would have to deal with the company directly to be removed.

Reasons for landing on the blocklist

  • Recipients of the email(s) being sent from the IP have reported it as spam using the spam button in their mailbox.
  • The IP has been reported to have sent malware or a virus.
  • Open relays or catch-all services are enabled. 
  • The owner has marked the IP address as dynamic and leased it out to their customers.

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