Blacklists you should worry about for email delivery.

email validation

In email marketing, a blacklist is a collection of IP addresses or domains that have been associated with the sending of unsolicited commercial email or SPAM. They are run by private individuals or anti spam organizations and listing criteria varies greatly. Once triggered and you’ve been listed, the best recourse is to validate your database.

Network administrators, ISP’s (Internet Service Providers) and mail admins use information from blacklists to prevent junk or unwanted email from being delivered. When an IP address or domain is listed on a significant blacklist delivery rates and inboxing will take a nosedive.

There are hundreds of blacklists. Most will not impact your delivery very much, but there are a few that will kill your marketing campaign. We go over the major blacklists below.


Spamhaus is the largest and most respected blacklist in the world. A listing at this blacklist will kill email delivery, usually dropping it to around 50%. Many ISP’s all over the World use Spamhaus to prevent bulk email from reaching their users.

The definition of SPAM according to Spamhaus is:

The word “Spam” as applied to Email means “Unsolicited Bulk Email”

Unsolicited means that the Recipient has not granted verifiable permission for the message to be sent. Bulk means that the message is sent as part of a larger collection of messages, all having substantively identical content.

A message is Spam only if it is both Unsolicited and Bulk.

Unsolicited Email is normal email
(examples: first contact enquiries, job enquiries, sales enquiries)

Bulk Email is normal email
(examples: subscriber newsletters, customer communications, discussion lists)”

There are different Spamhaus listings which we’ll go over in a minute.

Spamhaus SBL

Many times this is a manual listing affecting one IP address or it can be expanded to include an entire range, network, or company depending on the severity. Volunteers from around the World monitor sender activity and collect data, which is then used to list IP space. A common reason for a listing is sending email to an spamtrap address under Spamhaus’ control.

Spamhaus does not accept feedback from recipients. Although they have been known to work with certain networks and system administrators in order to curb serious SPAM issues.

A listing on Spamhaus is an email delivery and business killer. Most hosts or ESP’s are unable to provide service to anyone who triggers Spamhaus listings.


Impact: SpamCop can have a significant impact on B2B email campaigns.

The two ways to get listed on SpamCop are:

  1. Your email hit an email address (spamtrap) under SpamCop’s control.
  2. A SpamCop user reported the email.

Most of SpamCop’s spam traps are previously valid addresses that have not been active for 12 months or longer. has a large number of these SpamCop traps and continually update new traps them with cooperating ISP’s and ESP’s.

SpamCop is a dynamic blacklist, listings typically resolve themselves within one business day. There is no action you will need to take to action the delisting with SpamCop, the Privacy Team researches every SpamCop listing and will request delistings when an alert is received that an IP is listed. If your email activity triggered a SpamCop listing it likely means that you have a list management problem that should be addressed.


Manitu is a German blacklist and has a wide footprint in Europe. Email senders with European audiences tend to encounter this blacklist most frequently. Manitu is not used by North American ISPs to inform blacklist decisions but if you’re sending to Europe a listing could be problematic.

Listings are automatically activated when a sender mails to a Manitu owned spam trap address.

By working with this blacklist the Privacy Team is usually able to identify the customer and let them know that email activity from their subscription triggered a listing. Because Manitu operates solely on the use of spam trap addresses, getting listed by Manitu is a clear indication that senders need to audit their mailing lists.


Impact: The impact of a listing at SORBS is very minimal.

SORBS uses several methods to identify potential spammers. Most of their lists use spam traps to identify problematic senders. But SORBS will also list a sender based on their own user complaints, if SORBS administrators have received spam from the sender, or if they identify other high-level sending behavior patterns characteristic of spammers.

Next steps: The Privacy Team monitors SORBS activity and makes delisting requests for Marketo IPs as necessary. Oftentimes, SORBS will refuse to delist within a certain timeframe based on the severity of the issue. Sometimes this can be up to several weeks.


The impact of a listing at UCEPROTECT is very minimal, though the blacklist has a greater footprint in Europe. The organization does not have a good reputation in the industry because they charge senders to request delisting.

UCEPROTECT lists IPs that send mail to their spam trap addresses.

We ignore these listings because the only way to have them removed is to pay. The pay-to-delist model is not well respected in the email industry so UCEPROTECT has a very limited reach.

ISP Blacklists

Some ISPs have their own blacklists that they use to inform blocking decisions. A few examples are Comcast and Verizon. If your IP is being blocked by one of these networks, and those networks have a large presence in your lists, a block of this kind could have a noticeable negative impact on delivery. Usually blocks at Comcast and Verizon are resolved within less than 24 hours of a delisting request.