Author: Helen GeibBest Practices

Leveraging Email Domains for Data Filtering

Email discovery image

Email discovery is at the nexus of high volume and low relevance. Businesses generate a tremendous quantity of email that must be sifted in litigation to find a relatively small proportion of relevant messages and attachments. However, while email discovery can be burdensome and expensive, it is also necessary and important, and will be for as long as email remains the primary means of business communication.

Fortunately there are eDiscovery tools and strategies for making email review more manageable. One of the less well known, and consequently underutilized, options is filtering using email domains. Domain-based filtering is a cost-effective means of culling non-relevant email and a useful tool for prioritizing review of highly relevant communications.

What Is the Email Domain?

The email “domain” is the part of the email address that follows the @ symbol. In the email address, “photosubmissions” is the “local part” and “” is the domain. The metadata fields FROM, TO, CC and BCC contain the email addresses for the sender and recipients. Since the domain is intrinsic to the email file there is no added cost associated with collection or extraction.

Why Filter Using Domains?

Many email domains are known from the outset of the case to be either potentially relevant or definitely non-relevant. The third group of unknown or undetermined domains can be investigated as part of the filtering protocol, or the emails in those domain groups can simply be included in the review set by default.

Email domains are a quick and easy way to cull such obviously non-relevant message categories as:

  • Online shopping sites
  • Customer loyalty rewards programs
  • Travel-related websites and notifications
  • News and sports sites
  • Spam and phishing emails
  • Internet gambling
  • Government or court notifications
  • Professional and business associations
  • Newsletters, digests and other mailing list alerts
  • Social media notifications

Conversely, domains are a cost-effective means to locate and prioritize review of obviously relevant communications; for instance, the email domain of the opposing party or the email address of an important third-party witness.

In case with narrow, well-defined issues, reverse culling may even be appropriate. In reverse culling, only the domains marked as relevant are retained while all other domains are excluded from the review set.

Identifying Domains to Use as Filters

The first step in domain filtering is to ask your eDiscovery vendor or litigation support staff to export a list of the unique domains in the email collection.

The second step is to mark the list for relevant, non-relevant and unknown domains. These are three possible approaches, which can be employed singly or in combination:

  1. Someone on the eDiscovery team reviews the list and identifies known domains (e.g., the client’s own email domain,, any “.gov” domain). That person may be a lawyer, paralegal, project manager or eDiscovery consultant.
  2. The mailbox owners can be tasked with marking up the list. As the people most familiar with the content of their own email they can make the most comprehensive review. Another advantage to this approach is that there’s no out of pocket cost for the client; however, it obviously does require employee cooperation and time, which may not be possible or practicable.
  3. Have the document review team mark up the list on a rolling basis as they encounter new domains in the course of making the substantive responsiveness review. Filtering is likewise done on a rolling basis, sometimes called dynamic culling.

Once the domain list has been finalized, the final step is to apply the appropriate filters to the email dataset. Emails with non-relevant domains are culled and emails with relevant or unknown domains move forward to the next stage of the review process. In some cases, batch-tagging some emails as responsive based on their domain will be a viable, cost-saving review strategy. Additionally, by adding email threading, relevant and unknown domains can be strategically assigned, resulting in further cost-saving efficiencies.

Filtering Using Email Addresses

Domain filtering is an efficient way to make a first pass review of corporate email; that is, email generated by businesses, organizations and the government where multiple related email addresses are grouped by a single domain. It is not useful for personal webmail accounts (e.g.,,, since with webmail the addresses sharing the common domain are unrelated.

Following the same process as domain filtering, a supplemental targeted review may be made of the unique webmail addresses within an email collection. While full addresses can be a means to identify relevant communications, they are more commonly used to identify and exclude sensitive non-relevant messages from the review set. Correspondence with family and private financial communications are typical examples.

Email Domains and Privilege Review

Finally, email domains can be a useful tool to identify potentially privileged communications and segregate them for later privilege review. Instead of marking the email domain list based on relevance, it is used to identify outside counsel, consulting experts, eDiscovery vendors and other litigation consultants. As well as the efficiency gains from using domain filtering, batch-tagging using email domains is a safety net to catch messages to and from people whose email addresses don’t appear on a privilege search list; for instance, support staff with limited client contact.

Email volume continues to increase. Fortunately there are strategic ways to cull and organize email communications for an efficient and cost-effective review. Because many domains are known to be either responsive or non-responsive leveraging email domains provides an easy first step in culling and organizing large sets of data. To take this process a step further and create even more efficiencies and cost savings, add email threading. Email threading groups related emails together, allowing entire conversations to be assigned to a single reviewer.

Domain sorting is just one of many tools for email discovery. For more on strategies and tools to make the entire review process more efficient, more effective and less costly, see my Inside Counsel article The email discovery lifecycle’s four stages – and what to do at each step.


Helen Geib is General Counsel and Practice Support Consultant for QDiscovery. Prior to joining QDiscovery, Helen practiced law in the intellectual property litigation department of Barnes and Thornburg’s Indianapolis office where her responsibilities included managing large scale discovery and motion practice. She brings that experience and perspective to her work as an eDiscovery consultant. She also provides trial consulting services in civil and criminal cases. Helen has published articles on topics in eDiscovery and trial technology. She is a member of the bar of the State of Indiana and the US District Court for the Southern District of Indiana and a registered patent attorney.


This post is for general informational and educational purposes only. It is not intended as legal advice or to substitute for legal counsel, and does not create an attorney-client privilege.


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