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Blocklists and Removal Handling
Answer ID 9014   |   Last Review Date 08/14/2019

What is a Blocklist and what is Oracle doing to handle them?




Blocklists are lists of mail servers or domains that have been identified by a blocklist provider as being a source of spam.  There are dozens of blocklists across the Internet; however, only a handful are considered reliable enough for widespread usage.

The criteria for appearing on blocklists are different for each list, and may be fact-based or completely arbitrary, making it sometimes challenging to get removed from blocklists.  Given the unique nature of each blocklist, appearance on a blocklist may or may not present meaningful deliverability challenges.


A blocklist is a list of mail servers (or domains) that have been identified by an individual or group as senders of spam. Some lists are compiled automatically, while others are manually compiled by individuals. Blocklist services range from respected and formalized groups to one-man operations, sometimes self-appointed vigilantes of the email world. Both methods can be extremely arbitrary and often result in "false positives" (e.g. email desired by the recipient that does not get delivered due to filtering or blocking).

There is no single reason a sender may get listed on a blocklist. In general, blocklisting services target companies perceived to be sending spam. Typically, this is because the group or individual maintaining the blocklist has received unsolicited email from the listed server. Some public groups, such as SpamCop, base their listings on the number of complaints issued from consumers. Even one complaint can cause a block to be issued. There also is no one single source for blocklists, making it difficult if not impossible to identify every blocklist currently active.

Some public blocklists are utilized by a significant number of ISPs and corporations but most do not seriously impact overall deliverability. The important thing to remember is that a public blocklist can only affect an email marketer if that mailer delivers to a domain that uses the blocklist to filter incoming email. A blocklisting service does not have the ability to block your email without the full cooperation of the administrator that manages incoming email for those recipients.  Senders should keep this in mind if they find they are listed on a smaller blocklist, in particular.  In these instances, it is most likely that only very few (or zero) members of a contacts list will be impacted by the blocklisting.

Many less recognized blocklisting services do not have a mechanism for resolving email block disputes. Even services that are more widely recognized and used by large companies have limited resources to respond to challenges from legitimate email marketers. This makes getting removed from a blocklist a slow and resource-intensive process for everyone concerned.

Private and Public blocklists

There are over 200 publicly available lists currently being used by smaller ISPs and corporations to protect their systems and their users from unsolicited bulk email (spam). Being listed in one of these lists will prevent email from being delivered from any listed servers to the users of these blocklists.

Private email blocking systems are those developed and maintained internally by corporations or ISPs such as AOL, RoadRunner and CriticalPath, whereby they create their own rule set or list of IP addresses or servers they choose to block. ISPs such as Hotmail and Earthlink/Mindspring use both their own home-grown blocklists/filtering methods and a 3rd party email filtering company for their customers.

Major blocklists

The Spamhaus Project operates lists which have the biggest deliverability impact on most senders.  It runs the Spamhaus Block List (SBL) and the Exploits Block List (XBL), and maintains the Register of Known Spamming Operations (ROKSO). A team of volunteers located all over the world maintains the Spamhaus Project. The SBL is widely used and protects as many as 40 million mailboxes.

The SBL is a real-time database of IP addresses of verified spam sources (including spammers, spam gangs and spam support services), maintained by the Spamhaus Project team and supplied as a free service to help email administrators better manage incoming email streams. This is one of the more respected blocklists and a listing on the SBL can significantly impact deliverability.

The XBL is a database of IP addresses of exploited machines. It is currently a mirror of the data from another blocklist (the Composite Block List (CBL) from This list is a technical list designed to catch and stop infected or compromised servers from sending spam.

The Register of Known Spamming Operations lists hard-core spammers who have been kicked off 3 or more Internet Service Providers.

SpamCop lists spam sources based on reports from users of the SpamCop reporting system. SpamCop attempts to estimate the level of spam/legitimate traffic from each address reported. SpamCop is highly aggressive, and their processes result in numerous false positives, typically legitimate list traffic running COI (confirmed opt-in). Because of this, SpamCop is not widely used at major ISPs, and a listing does not have a large impact on the average mailing list.

Oracle Impact

Oracle is a devoted practitioner of permission-based marketing, utilizing industry best practices for all clients. With false positives and the arbitrary nature of some small, independent blocklisting services, keeping off 100% of all available blocklists may not be possible. Our goal is to minimize email blocking for all our clients, and our entire staff is dedicated to that goal.

Oracle has developed a blocklist monitoring tool that regularly checks each client's IP address and then sends out an email alert to our deliverability monitoring team with the status. The lists that Oracle monitors can be changed or adjusted at any time. We developed the current list by determining which blocklists have the most market share and are mostly widely used. It is typically only these blocklists that could have a significant impact on a client's overall email deliverability. We take blocklisting very seriously and work with our clients to ensure that they stay off blocklists.

Oracle has an internal team devoted to deliverability and ISP relations, which includes relationships with the better-recognized email blocking groups and blocklists. We also utilize external resources, including email industry associations such as the ESPC and other professionals, to resolve blocklist-related issues. Although Oracle can help clients improve email deliverability, eliminating false positives is an ongoing challenge for all email including legitimate email marketers and ESPs.

For additional information, refer to the following:

Answer ID 2800: Bounced E-mail message handling

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