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A blocklist is a database of IPs and domains that have been flagged for possibly sending spam or malicious mail. Mail servers and providers will use different blocklists to scan for senders who may violate their email policies and reject mail to keep their users safe.

Blocklists 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 blocklist and provides real-time anti-spam protection.
  • SpamCop – Another widely used blocklist that is primarily made by reports from its user base.
  • Invaluement - Another strong and widely impactful blocklist. Easily plugs into corporate email filters, and is known to be used by Mimecast security filtering software.
  • Other RBL lists – There are over 230 other “real-time block lists” (RBL).

Causes and Effects

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

Cause Explanation
Spam Trap triggering Many blocklists have various “Spam Traps” spread across the internet. These traps can be a real email that the user has long since left, but the blocklist 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 blocklisting and usually has the most negative effect. One of the most effective ways to remove spam traps is to run them through a cleansing service. Learn more about list cleansing and list verification here.
Reported Spam (Unsolicited Commercial Mail) Depending on a sender’s and receiver’s location, Unsolicited Commercial Mail (UCM) is illegal in many jurisdictions. The email industry has decided that these emails are no good overall, and those who send them will be harmed in both reputation and with the possibility of being added to blocklists. 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. Blocklists 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 blocklist 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 blocklist listing.
Technical Issues Ignoring a proper technical setup and attempting to send repeatedly will cause a blocklisting.
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 blocklist.

There are several effects that a blocklisting can cause. IT departments and Postmasters of domains often subscribe to blocklists, 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 blocklistings on domains into their SEO calculations.

Blocklist Ranges

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

  • Domain blocklistings 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 blocklisted domain, your inboxing and delivery will also be affected.
    • Act-On does not monitor clients for domain blocklistings 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 blocklistings in email.
    • Any Act-On customer will have their IPs monitored for blocklistings, 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 on email senders and Email Service Providers.
    • These blocklists 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.

Hidden BlockLists

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

There is no public website to query whether you are on a blocklist of many email providers or security filters. 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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