Reconstruct the change
Identify the old and new services, relevant dates, the account administrator and any export, migration or deletion steps. Determine whether the problem concerns missing content, altered representation, inaccessible archives or uncertainty about the completeness of the transfer. Keep those questions separate.
Potential evidence
Migration packages, old mailbox exports, local client stores, backups, archive systems and recipient-held copies may contain different parts of the history. Logs and migration reports may explain exclusions or errors. Preserve existing packages before attempting restoration or conversion, especially where they are the only remaining source.
What an assessment can establish
A comparison may identify messages present in one set and absent from another, but it cannot assume that either set represents the full historical account. A new hold does not guarantee restoration of already purged data. Available access, encryption and source condition can limit examination. Begin with the change chronology and the inventory of retained material. Possible outputs include a source map, feasibility opinion, comparison schedule and an explanation of migration-related differences that should not be mistaken for deliberate alteration.