You may occasionally see a bounce that has been recorded for an email you sent, and the reason for the bounce refers to a blocklist. Bounces are a normal part of sending email, and blocklists are a normal part of the way that bulk email works. ClickDimensions believes in transparency, providing our customers with complete visibility into their delivery rates.
Many email marketing systems will tell you that some percentage of your emails bounced, but they may not show you all the reasons why. Because every email marketing system is subject to bounces caused by blocklists, our goal at ClickDimensions is to provide you with the best overall delivery rates possible and to minimize bounces wherever possible.
What is a blocklist?
In an attempt to prevent unsolicited email (spam) from clogging people's inboxes, many email services use a variety of tools to determine if an email is legitimate and desired by the intended recipient. One of the tools they use is an RBL, for Real-Time, or Remote BlockList. An RBL is a list of known or suspected IP addresses that are considered to be sources of spam. There are both IP-based blocklists and domain-based blocklists.
How many blocklists are there?
There are hundreds, if not thousands, of RBLs in the world. Some of them are maintained by large companies, and some are open source projects run by volunteers. There are a few legitimate and respected blocklists but there are many more blocklists that are poorly supported, use dubious practices, and have a low to zero impact. You can check the most high-quality, impactful lists, and we recommend using their own tools (Spamhaus, SpamCop, SORBS), if you decide to do that.
How do blocklists work?
Most IP blocklists are based off spam trap email addresses where if an email is sent to a spam trap address owned by the blocklist they will list the IP responsible for the email sent. The more legitimate blocklists will use algorithms to monitor and measure email that flows through their systems, ranking the volume, frequency, and content of the email messages and tracing the messages back to the source. While others will list an IP based off a single email to a spam trap.
Other blocklists use manual reporting mechanisms where participants report suspected IP addresses to the blocklist. If enough people report the IP address, the blocklist may add it to their list of known or suspected sources of spam. Blocklists themselves do not block emails but the email systems that reference the blocklists do the blocking.
Regardless of the method of compiling a blocklist, mistakes are sometimes made and legitimate sources of email can be added to the list. In turn, the email service providers and spam filtering applications that reference a given blocklist may begin blocking an email based on faulty information. (Of course, sometimes, the IP address really is a source of unsolicited email if it has been compromised by a spammer or if one person using a shared IP address for email decides to send a lot of junk.)
What can email senders do about blocklists?
Blocklistings are preventable by keeping marketing lists updated and clean by using permission-based marketing. Ensuring the integrity of the addresses you collect (Did they confirm their opt-in? Are they current customers?) will prevent you from acquiring spam trap addresses. A regular audit of email addresses to ensure that they are engaging with your emails and removing the ones that aren’t, will get rid of spam traps currently in your list. For more information about how to avoid spam trap addresses, read our Spam Trap article.
Some blocklists will list the IP responsible for sending the email and also list the sender’s domain. If the domain is listed it will impact all emails sent using that sending domain—not just email sent through ClickDimensions.
If a sender is responsible for a trend of blocklistings, we will reach out via a Support ticket and ask that list quality is addressed. Repeated blocklisting will result in a suspended account.
Regarding purchased and 3rd party lists—using purchased lists, rented lists, or 3rd party lists where the sender is not receiving the permission directly is against ClickDimensions Terms of Service. They almost always contain bad quality addresses including spam traps.
What can email providers do about blocklists?
Due to the imperfect nature of blocklists, many blocklist providers offer a way for an email administrator to request "de-listing"—in other words, to ask the company maintaining the blocklist to reconsider, remove the IP from the list, or to provide additional information about how it got listed. Each blocklist provider has their own mechanism for handling these requests; although some blocklist providers may have no mechanism to handle correction requests. Sometimes the process of de-listing an IP address is automatic when a de-listing request is received, sometimes the listing expires automatically, and sometimes the blocklist provider requires several rounds of discussion and proof before removing a blocklisted IP address.
ClickDimensions, like all companies that provide email marketing, has a deliverability team that works around the globe, monitoring the major blocklists around the world and working with the blocklist providers to keep our IP addresses off of their lists. By the time you see a bounce reported in your CRM due to a blocklist, chances are that our deliverability team has already begun working with the blocklist provider to request de-listing. In most cases, your marketing email is not affected by a blocklisting, but occasionally a small percentage of your overall email might be impacted by a blocklist. In these situations, you can rest assured knowing our deliverability team is working with the blocklist provider, or if you have a question or concern about a blocked email due to a blocklist, you can talk with our support team about it to find out more.
What else can cause an email to be rejected?
Blocklists are one method that email providers and mail servers might use to evaluate the legitimacy of an email, but there are other methods used as well. Some large corporations maintain their own internal blocklists based on their user’s interactions with emails or on other filtering mechanisms. Additionally, your recipients may use their email application, such as Outlook, to "block" future emails from you or your domain by moving your messages to their junk folder. Some antivirus applications will also scan a recipient's inbox for suspicious email and occasionally mistake a legitimate email for a phishing attempt. (This can happen on the recipient's computer, or on their corporate network.)
In short, email delivery brings many factors into play, some of which you can help to control (for example, by sending timely, relevant email only to those who have given you permission to email them), some of which are in our control, and others which are beyond the control of the sending parties.
Some Blocklists to Note
Spamhaus
Spamhaus has multiple blocklists and they are all highly regarded and impactful. Some of the blocklists are automated while others are reviewed by a human. We take Spamhaus listings very seriously. Any sender that causes an IP to be listed on Spamhaus will be immediately suspended until we can resolve the issue.
SpamCop
SpamCop, owned by Cisco, is also a blocklist that we consider impactful. SpamCop will list an IP for 12–48 hours or longer if there are repeated instances of spam. SpamCop is one of the blocklists that Mimecast references (if you use Mimecast as a spam filter you may want to whitelist our IPs). If a sender is responsible for listing an IP on SpamCop, we will reach out and request that they address the issue.
SORBS
SORBS, owned by Proofpoint, is medium-to-low-impact. They list an IP for a minimum of 48 hours and longer depending on the amount of spam traffic they are seeing. If a sender has a trend of being listing on SORBS, we will reach out and request that they address the issue.
UCEProtect
UCEProtect is a low to no impact blocklist based out of Germany. It is not widely used at all, but is seen in use by very few organizations in Europe. They are themselves considered suspect RBL providers. They list an IP for a minimum of 7 days and do not offer a means to delist. We recommend you routinely ignore their listings.
MagicMail/Linux Magic/MIPSpace
MagicMail uses the MIPSpace blocklists. This list is not based on traditional “spam” traffic but any and all bulk email traffic. If an IP is used to send bulk mail, even fully-permissioned bulk email, MIPSpace will list the IP. Only non-bulk email senders are able to delist their IP.
Recipients that use MagicMail as their spam filter are not able to mark a sender as “safe” as in Outlook. They cannot overwrite the rules.