Preserve both sides
Identify the source system, destination, migration method, dates, options and error reports. Retain available pre-migration exports or backups separately from the current mailbox. An exported file is itself an artefact of a process; record which stage it represents. Do not overwrite an older copy while trying to reconcile it.
Compare the relevant fields
Possible differences include folder or label mapping, local file dates, formatting, duplicate handling and supported message properties. Examine the actual process and files rather than assuming any particular transformation occurred. A different whole-file hash is expected when the representation changes; it does not determine whether the disputed sentence or attachment changed.
Make unexplained gaps visible
Build a documented comparison using suitable identifiers, content and attachment relationships. Review excluded items and failures, and separate explained transformation from an unresolved discrepancy. The aim is to establish which changes matter to the evidential question. A successfully completed migration does not necessarily prove that every historical item was transferred. If the old environment no longer exists, describe the remaining comparison limits and assess independent correspondents or archives where access is authorised.
Sources & further reading
Primary references support the technical explanation. They do not establish D3 validation of a particular method.