Useful relationships
Message-ID is intended to identify a message. In-Reply-To and References can express relationships to earlier messages. Applications may also use subjects or their own conversation properties. An examiner records the actual fields and the grouping method rather than treating a mail application's conversation view as a complete historical record.
A fictional example
A reply from analyst@example.org may cite an earlier message identifier even when the earlier message is absent from the supplied export. That is a lead to investigate, not a substitute for the missing content. Quoted text may preserve part of the earlier wording, but someone can edit quotation text before sending a reply.
Where reconstruction stops
Duplicate, missing or malformed identifiers and conversions can complicate grouping. Matching identifiers alone do not show that the files are byte-identical, nor that the named sender wrote them. Retain the independent items and document uncertain relationships. Request other participants' copies where authorised, and distinguish a directly examined message from content reproduced only in a later quotation. A defensible thread should make gaps and competing versions visible instead of silently merging them.
Sources & further reading
Primary references support the technical explanation. They do not establish D3 validation of a particular method.