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Email Threading: Streamlining Discovery Review

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For years, email has been the dominant form of business communication. The average office worker sends approximately 9,000 to 15,000 emails per year.1 Multiply 15,000 emails across multiple personnel and multiple years, and that leads to an enormous amount of email flowing into review for a lawsuit or investigation. And, as we all know, many of those emails are replies and forwards containing repetitive information that appear multiple times in our inboxes. Reviewing each individual email in those email chains or “threads” can be time-consuming and costly. Email threading can help solve this problem.

Email threading technology identifies all of the emails in a communication chain so that a reviewer can view them as one coherent conversation, generally in chronological order with the most recent and most inclusive emails first. Each “branch” of an email chain — that is, versions of the email communication that contain unique content, different recipients or unique attachments — are reviewed separately. And in most instances, only the most inclusive email — which contains all of the earlier replies — is presented as a part of the files to be reviewed while the less inclusive, earlier emails are suppressed from the review.

By reviewing only the most inclusive emails, the process becomes much more efficient. Counsel can more effectively develop a better understanding of the content and flow of the conversation by reviewing the multiple related emails in sequential order as opposed to a jumble of unorganized and duplicative emails. Even in smaller cases, the elimination of duplicate emails can result in a 25 percent to 60 percent reduction in document review cost and time.2

Despite its many benefits, there have been some concerns raised about the use of email threading in discovery.

Some argue that email threading could inadvertently exclude important metadata. This took place in In re Actos Antitrust Litigation, where the court found that the production of threaded emails resulted in the exclusion of metadata from earlier emails in the chain, deterring the plaintiffs’ ability to search for emails by date and see who had been blind-copied on earlier emails.3

Another concern is how email threading works with privilege logging. There is currently a split of authority as to whether a privilege log should separately itemize emails within an email chain or treat the chain as a single entry. Some courts favor itemized privilege logs, while others have found that a party only needs to include one entry to identify withheld emails that constitute an uninterrupted email chain. Even the drafters of the eDiscovery think-tank, The Sedona Conference, acknowledged that there is no consistency in this area to date:

“The drafters of this Commentary are not aware of any decision addressing whether emails suppressed from review through email thread identification technology must be separately logged. These decisions assessing the need to individually log emails in an email chain would logically be applied to the question of whether individual emails suppressed via threading also need to be individually logged. Courts reaching decisions consistent with the Rhoads Industries v. Building Materials Corp. of America cases require logging of each individual email in a chain as a separate document being withheld. On the other side of the question, courts reaching the decision consistent with Muro v. Target allow multiple emails in the same chain to be logged as a single entry, provided that all the parts of the communication in the email chain were properly privileged, or nonprivileged portions were otherwise produced.”4

But these concerns can be easily addressed. Parties can negotiate and establish a comprehensive Electronically Stored Information (“ESI”) protocol before data production begins, which many courts have encouraged to ensure critical metadata is not lost in the process of email threading. By establishing a protocol in advance so that both parties are aware of what is being included, there is less of a chance of pertinent data being excluded. The drafters of The Sedona Conference commentary have encouraged such protocol: “The parties should discuss email threading, and its implications on the information that will be reflected in the privilege log, or what additional information may be helpful to provide about the metadata of suppressed emails, early in the case, and in any event, before privilege logs are created and produced.”5

Courts can also address these concerns by ordering parties to produce the most inclusive version of an email thread, but requiring the production of each version of the email thread that includes unique information.6 In addition, some courts can include their instructions on email threading during review and privilege logging as a part of their standing orders.

With these fixes, most courts have found that email threading reduces unduly burdensome costs in reviewing emails7 and have increasingly accepted email threading.8 The key as always is to consider the many options in advance, address any concerns in a meet and confer, and then document all decisions about the approach for handling email threading in a well-drafted and technically correct ESI protocol. Always consult with your technology experts to ensure that these decisions are appropriate to your data set and to your litigation support technology.

1James Norquay, How Many Emails Are Sent Per Day In 2024?, Prosperity Media (Dec. 20, 2023),,to%20the%20average%20office%20worker.

2Wendy Cole, No Matter is Too Small – the Case for Email Threading, Heuristica Discovery Counsel LLP, Nov. 5, 2018,

3In re Actos Antitrust Litig., 340 F.R.D. 549 (S.D.N.Y. Mar. 30, 2022).

4Sedona Conference, The Sedona Conference Commentary on Privilege Logs 24 (Feb. 2024 Pub. Comment Vers.),

5Id. at 25.

6In re Facebook, Inc. Consumer Priv. User Profile Litig., No. 18-MD-2843 VC (JSC), 2021 WL 10282215, at *31 (N.D. Cal. Sept. 29, 2021).

7Allen v. Yertle Operations LLC, 70 Misc. 3d 934, 139 N.Y.S.3d 758 (Sup. Ct. Westchester Cty. 2020).

8Rabin v. Pricewaterhousecoopers LLP, 16-CV-02276, 2016 WL 5897732, at *1 (N.D. Cal. Oct. 11, 2016) (“Where multiple email messages are part of a single chain or “thread,” a Party is only required to produce the most inclusive message and need not produce earlier, less inclusive email messages or “thread members” that are fully contained, including attachments and including identical senders and recipients, within the most inclusive email message.”); In re Meta Pixel Healthcare Litig., 2023 WL 4361131 (N.D. Cal. 2023) (“This is particularly true here where a party is only permitted to produce the most inclusive message, and avoid separately producing the earlier messages in the email thread.”).

Key Contacts

Senior eDiscovery & Litigation Support Director
Jennifer Williams

This information is provided by Vinson & Elkins LLP for educational and informational purposes only and is not intended, nor should it be construed, as legal advice.