A blacklist is a database of IP addresses or domains that have been associated with the sending of unsolicited commercial email or spam.  Internet Service Providers (ISPs) and business email networks use information from blacklists to filter out unwanted email.  As a result there can be a drop in inbox delivery rates or overall delivery rates if the IPs or domains involved with sending email are listed on a blacklist.

Marketo’s Email Delivery and Compliance team monitors blacklist activity on our IPs and domains daily. When we are alerted to a listing we reach out to the blacklist, attempt to identify the sender that triggered it, and work with the blacklist organization to get the listing resolved.

There are thousands of blacklists out there most will not have a significant impact on your delivery rates. Below we have compiled a list of the blacklists that our customers most commonly encounter.

Tier 1 Blacklist


Impact: Spamhaus is the only blacklist that we categorize as a tier 1 for a reason: it has by far the greatest impact on delivery of all of the blacklists. It is the most well-respected and widely used blacklist in the world. A listing at Spamhaus will have a negative effect on your ability to deliver emails to your customer’s inbox and can cause bounce rates of over 50%.  Evidence suggests that most of the top North American ISPs use Spamhaus to inform blocking decisions.

How it works: Unlike many blacklists, Spamhaus lists senders manually. This means that they are proactively watching sender activity, collecting data, and basing their listings on a number of variables. Most commonly senders are listed for mailing to spam trap addresses that Spamhaus owns. Sometimes Spamhaus will list senders based on recipient feedback as well.

Next steps: Our team monitors closely for Spamhaus listings. When we see a listing we immediately alert the customer and contact Spamhaus to start the remediation process. Listings last until Spamhaus is satisfied that the offending sender has taken the appropriate steps to mitigate the problem.


Tier 2 Blacklists


Impact: SpamCop is not used by any of the major North American ISPs to inform blocking decisions but it makes it to the Tier 2 list because it can have a significant impact on B2B email campaigns.

How it works: SpamCop lists IPs for one of two reasons:

      1. Either the email hit SpamCop spam trap addresses OR
      2. A SpamCop user has reported the email unwanted.

Most of SpamCop’s spam traps are previously valid addresses that have not been active for 12 months or longer.

Next steps: 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.


Impact: 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.

How it works: Listings are automatically activated when a sender mails to a Manitu owned spam trap address.

Next steps: The Privacy Team researches and requests delisting when an alert is received that a Marketo IP is listed.  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.


Tier 3 Blacklists



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

How it works: 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.



Impact: 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.

How it works: UCEPROTECT lists IPs that send mail to their spam trap addresses.

Next steps: 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. Marketo monitors for this type of ISP specific blacklisting and the the Privacy Team works to resolve these as soon as possible. Usually blocks at Comcast and Verizon are resolved within less than 24 hours of a delisting request.


Additional Resources:

Blacklist Deep Dive

Abuse Report Deep Dive

What is a spamtrap, or spam trap, and why does it matter?

Blacklist remediation

Blacklist resolution flowchart

Successful lead reconfirmation

What is a blacklist?

Labels (2)