If email header fields are not set on an Analytics profile, Relativity Analytics will try to run email threading on the actual header of an email. However emails that have non-standard headers cannot be analyzed. You can see a chart of email header formats supported in Relativity 9.6 posted here.
The Tip of the Night for July 28, 2018, discussed how to find an email header in an Outlook message and mentioned common Mail User Agent (MUA) formats.
Let's review this example on an email header taken from a message in the Enron Email data set.
Date: Sat, 26 Jan 2002 07:58:40 -0800 (PST),Sat, 26 Jan 2002 07:58:40 -0800 (PST)
Content-Type: multipart/mixed; boundary="----=_Part_16645_20266206.1241755557454"
cc: "Williams III", Bill </O=ENRON/OU=NA/CN=RECIPIENTS/CN=Bwillia5>
subject: epe model for 01/25/02
filename: bert meyers 6-25-02.PST
folder: \ExMerge - Meyers, Albert\Sent Items
date: Sat, 26 Jan 2002 07:58:40 -0800 (PST),Sat, 26 Jan 2002 07:58:40 -0800 (PST)
Focus on the standard email header fields: date; subject; from; to; cc. In this example the name of each field is followed by a colon followed by a space and then the value of the field follows on the same line. The format is the same as this one given in the Relativity Analytics guide:
Note that there are different formats for primary and embedded emails. Other standard email header formats may place field values on lines after the field labels, or use different field labels for the same type of values.
The embedded headers in an email thread will also be used for email threading Analytics and must be in standard formats as well. French, Chinese, Japanese, Spanish, German, Portuguese, and Korean email header formats are supported. Note that email headers in extracted text must come before any other text.
Analytics can run email threading on any of the fields listed below regardless of the order in which they are listed in the header.