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A blacklist is a publicly available source of information about an email sender that identifies a sender with an issue. This list usually corresponds to an inbox audience who will block incoming email traffic from that sender.

Blacklists come in many varieties and their effects and causes are widely varied. Some are far more severe than others.

  • SpamHaus – SpamHaus is the largest and most widely subscribed to blacklist and provides real-time anti-spam protection.
  • SpamCop – Another widely used blacklist that is primarily made by reports from its user base.
  • Other RBL lists – There are over 230 other “Real time Blackhole Lists” (RBL).

Causes and Effects

There are several ways to be placed on a blacklist, listed below by typical impact and range. There is some chance of inappropriate blacklisting, but as methods improve this is increasingly rare.

Cause Explanation
Spam Trap triggering Many blacklists have various “Spam Traps” spread across the internet. These traps can be a real email that the user has long since left, but the blacklist provider or ISP left on to track Spam; or they can be an email that was created for the sole purpose of catching marketers who scrape emails or buy/rent lists from a third-party vendor to use in commercial emailing.

This is the most common reason for a blacklisting, and usually has the most negative effect.
Reported Spam (Unsolicited Commercial Mail) Depending on a sender’s and receiver’s location, Unsolicited Commercial Mail (UCM) may or may not be legal (LINK TO JURSIDICTIONS); however, the Email Industry has decided that these emails are no good over-all, and those that send them will be harmed in both reputation, and with the possibility of being added to blacklists. Generally, low reputation or reports generated by end users cause these listings due to UCM.
Phishing Phishing is regarded as the most dangerous type of spam. Any attempts to mislead consumers with subject lines, content or from messages can be regarded as phishing. Blacklists for these events are very damaging to reputation.
Complaint Rate If a complaint rate (spam complaints/volume) reaches a certain level with some ISPs, a blacklist can trigger. These triggering events usually coincide with a general degradation of reputation at all ISPs and an increase in spam placement.
Admin Spam Trap If during a UCM someone sends to the admin of a spam trap list, they can also place you on the blacklist listing.
Technical Issues Ignoring a proper technical setup and attempting to send repeatedly will cause a blacklisting.
SEO/Domain Reputation If the reputation of a domain tied in some way to an email sender hits a low point, their email IP or domain can also be put on a blacklist.


There are several effects that a backlisting can cause. IT departments and Postmasters of domains often subscribe to blacklists, and if a domain is listed they will reject mail. Other ISPs and domains also use listings on the RBL to determine things like inbox placing. Google and other search companies figure blacklistings on domains into their SEO calculations.

Blacklist Ranges

There are 3 'ranges' of a blacklist: domain, IP, or IP Subnet (neighborhood).

  • Domain blacklistings only initially affect traffic on one domain, or emails tied to that domain.
    • This can reach further than expected – if you send an email referencing a blacklisted domain, your inboxing and delivery will also be affected.
    • Act-On does not monitor clients for domain blacklistings, unless a sender is subscribed to our Enhanced Deliverability Services.
  • Single IP listings generally only affect traffic coming from one IP address.
    • These are the most common blacklistings in email.
    • Any Act-On customer will have their IPs monitored for blacklistings, whether in the general monitored shared environment or using Enhanced Deliverability Services and a dedicated IP.
  • IP Subnet listings are the rarest type, but they have the most devastating effects to email senders and Email Service Providers.
    • These blacklists affect all the IPs on the subnet (neighborhood) in chunks of 8, 16, 32, 64, 128, or 256.
    • Act-On monitors all our IP assets for these listings.

Remediation (Getting off a Blacklist)

This depends on the blacklist – each one could have some differences. Some are as simple as going to a site and clicking a button, while other are more complicated. Around 2010, reaching out to the appropriate person via email or chat would get you taken off the list. This level of personal review has been largely removed from remediation, though it still exists in some cases.

More impactful blacklistings carry harsher penalties. These often increase with each additional blacklisting and can become year or multi-year blacklists with just a few listings. Example: SORBS, a blacklist owned by Proofpoint, has an increasing timer, generally explained in their FAQ. This new approach by several blacklists has also caused Email Service Providers of all types to take blacklists more seriously, as this significantly affects their inventories of IPs.

If you have been placed on a blacklist, please consult the remediation process page.


As well as public blacklists, there are also more hidden blocklists. These blocks are frequently triggered by reputation issues and can be tied to a public blacklisting. Some examples of blocklists are: Cloudmark, Proofpoint, Mimecast, Gmail, AOL, Microsoft, etc.

These lists are harder to combat and often require the help of a deliverability expert to diagnose and deal with on a long-term basis.

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