Blocklist Basics

An email blocklist is defined as any group of IPs or sending addresses that are suspected of email spam activity. Blocklists exist to protect users from spam messages and allow legitimate email to be delivered (and get the attention it deserves).

The first blocklist was created in 1997 by Internet Hall of Fame inductee (yes, that is a real thing), Paul Vixie. The “Real-time Blackhole List,” as he called it, was formed as a response to the overwhelming growth of spam that occurred during the Internet boom. With no government action on spam email taken until 2003’s CAN-SPAM Act, it was up to Vixie and other Internet titans to take up the battle. Since 1997, hundreds of other blocklists have popped up to help in the ongoing effort to curtail spam messaging.

There are two types of blocklists: IP blocklists and domain blocklists. IP blocklists are concerned with the source of the message (the sending IP), while domain blocklists are concerned with the source of the links inside the message. Typically, honest senders see more trouble with IP-based blocklists than they do with domain-based blocklists.

The Usual Suspects: IP Blocklists

There are several main blocklists you should be concerned about staying off of, as a sender, including the ones listed in this section.

Spamhaus: The Alcatraz of Blocklists

Want to keep a spammer up at night? Just mention the name “Spamhaus.” Using advanced tactics, Spamhaus protects hundreds of millions of inboxes from unwanted messages. Unlike many blocklists, which have easily accessible avenues for remediation, being listed on Spamhaus ensures a lengthy, complicated process for removal. Just like “The Rock” that once housed Al Capone, the best way to get out of a Spamhaus listing is to never get on it in the first place.

Senders can wind up on Spamhaus by sending to Spamhaus spam traps, or being reported by a receiver as a spammer. To avoid this, make sure you maintain clean data and practice opt-in only marketing campaigns. Spamhaus is used by many major ISPs, including Gmail; if you get listed, it won’t be easy, fast, or inexpensive to mitigate the drastic effects on your deliverability. You’ll need to come up with a plan, share it with Spamhaus, and prove to them that you’re following it.

SpamCop: Trapping Spammers Since 1998

One of the most active spam trap blocklists is SpamCop. While it’s possible to be listed on this blocklist by getting too many spam complaints registered against you, the most common way to wind up on SpamCop is by sending to one of its spam traps.

SpamCop maintains a wide network of spam traps (email addresses created solely to catch spammers). The best way to stay off SpamCop is to never purchase or rent data, and to perform regular list cleaning. SpamCop’s listings generally last for 24 hours, with automatic delisting available as long as you don’t send to another spam trap within 24 hours of the initial listing. During a listing you can expect to see increased bounces at the domains that participate in the SpamCop network, limiting the ROI of your marketing campaigns.

If you do find yourself listed on SpamCop, the best step you can take is to stop sending right away and immediately clean all your lists. While it’s not always perfect, list cleaning will remove known spam traps from your database.

Other Email Blocklists

As mentioned, there are hundreds of known blocklists in existence today. Other common IP blocklists include Invaluement (ivmSIP) and Truncate. The severity of a blocklist and its avenues for delisting vary by the source, but the best way to stay off blocklists is to maintain clean data, send only to engaged subscribers, and maintain low spam complaint thresholds.

The Usual Suspects: Domain-Based Blocklists

Spamhaus: The Domain Remix

No déjà vu here. In addition to its heavyweight IP blocklist, Spamhaus also features one of the most formidable domain blocklists out there as well. Like other domain blocklists, Spamhaus scans messages for hyperlinks directing the user to domains that are associated with, or considered a source of, spam. It’s very difficult to request or try to negotiate removal from SpamHaus’ domain blocklist; most listings will expire automatically after they cease to appear in spam. Similarly, domains are listed in DBL Zone (the blocklist) automatically, and they may re-list automatically after removal if they are re-detected.

Spamhaus is a colossus amongst mere blocklist mortals, and winding up on either Spamhaus blocklist (IP or domain) will greatly harm your company’s sender reputation. (You probably don’t want to be responsible for that.)

Other Domain Blocklists

Other common domain blocklists include URIBL and SURBL. These blocklists seek to pick up where Spamhaus leaves off, hunting down and blocking senders that link or redirect to domains that are associated with spam email messaging.

To stay off domain blocklists, keep your domain reputation intact by following the same sending best practices as for IP blocklists As the importance of domain reputation grows, soon your domain’s reputation is going to matter as much as a sender as your IP reputation does.

Conclusion: The Bottom Line on Blocklists

Although honest senders can wind up on blocklists, it’s important to reiterate that blocklists exist so that legitimate messages will be recognized as such, and permitted to progress to the inbox. Seeing as how data and time are dynamic functions, winding up on a blocklist is an inevitable eventual reality for the majority of senders. It’s up to us as marketers to ensure that our data is clean and up to date, and that we are sending only to people who want our emails in an effort to stay off blocklists and maximize marketing ROI.