About 90% of all emails sent worldwide each day are considered spam. Without blocklists and spam filters, it would be nearly impossible for anyone to read anything in their inboxes. This article explains what email blocklists are, who maintains them, how they work, and why an IP address or domain lands on one.
What an email blocklist is
A blocklist is a list of IP addresses or domains that are blocked for several reasons. These lists are kept in real time and are available to the public. Receiving mail servers query a blocklist to gather information on the reputation of an IP address or domain of an incoming email before deciding whether to deliver the message. Ending up on a blocklist does not necessarily mean the IP or domain is a spammer. It means the IP has been reported as a known source of spam.
Tip: MX Toolbox is a useful online tool for blocklist checks.
Email blocklist controllers
Third-party blocklist databases exist that email providers can query. The receiving mail server uses the information only to aid its decision on whether to refuse an email. The blocklist service does not determine whether 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, which should reduce false positives because users can remove their ISP's mail server from the list. |
| Spamhaus | The Spamhaus project is one of the most reputable blocklisting companies, providing real-time anti-spam protection for internet networks worldwide. |
| Spamcop | 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, it reaches out to the third-party blocklist database to find out whether the IP or domain is listed as a known source of spam. The blocklist returns a yes or no reply indicating whether the IP or domain is listed as a source of spam in the blocklist database for the provider being checked against.
Public blocklists
Public blocklists are shared via the internet and used by anyone to check IP addresses and domains, such as Spamhaus, Spamcop, and Surriel.
Private blocklists
Companies create private blocklists for their own security and spam filtering purposes, and these are not made publicly available. The methods a company uses to determine whether an IP or domain is listed on its private blocklist are proprietary. If you are blocked by one of these lists, you have to deal with the company directly to be removed.
Reasons for landing on a blocklist
- Recipients of the emails 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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