It's standard practice in electronic discovery to produce discoverable email messages with their attachments. Federal Rule of Evidence 106 encourages this practice, as it's the basis in federal evidence law for the 'rule of completeness'. FRE 106 states that, "If a party introduces all or part of a writing or recorded statement, an adverse party may require the introduction, at that time, of any other part — or any other writing or recorded statement — that in fairness ought to be considered at the same time." The production of document families in complete form is also encouraged by Federal Rule of Civil Procedure Rule 34(b)(2)(E)(i) which specifies that, "A party must produce documents as they are kept in the usual course of business or must organize and label them to correspond to the categories in the request.", implying both that emails needs to be produced with their attachments, and that they be sorted into different subject areas.
One of the leading cases on this issue is, Abu Dhabi Commercial Bank v. Morgan Stanley & co., Inc., 1:08-cv-07508-SAS-DCF (S.D.N.Y. Aug. 18, 2011). In this case three emails were produced without attachments, that were not included on the party's privilege log. The court in this case found that while there is not an 'ironcald' rule requiring attachments to be produced with parent emails it is the 'prevailing practice'. The special master, Jonathan Redgrave, who issued the 'Report and Recommendation on the Defendants' Motion to Compel SEI Investments Company to Procduce "Missing" E-Mail Attachments', stated that it is best practice for parties to discuss the production of email attachments prior to production.
In issuing his decision, Redgrave noted that attachments were missing in the production from more than 100 emails, and that the plaintiffs didn't indicate that emails had been collected from repositories where they had already been disassociated from their attachments - something the court viewed as a legitimate excuse. The court declined to find that the motion to compel was a 'fishing expedition'. It order SEI to produce the emails with their attachments, but also said that if the attachments could not be located without 'undue burden or expense' the plaintiffs could provide a detailed explanation instead.
The court then ordered the parties to meet and confer within 10 days in order to address whether the plaintiffs withheld any attachments as being irrelevant; whether privilege logs tracked emails and their attachments separately; and also to agree on the format of the production for both emails and attachments.