Identify the surviving source
A PST, mbox file, proprietary archive or old server backup may preserve useful correspondence even when the original account is unavailable. The filename alone does not establish format, completeness or accessibility. Record the source system, approximate dates and how the material was retained.
Preservation and feasibility
Keep the original storage and files intact. Work from an agreed copy and avoid repair or conversion over the only surviving material. Relevant prerequisites may include the software version, archive structure, companion files, passwords or decryption material, all handled through an agreed procedure.
What the result may show
Feasibility work can identify readable content, structural problems and missing components. Further examination may produce an inventory, extracted messages, an evidence schedule and documented conversion limits. A successful extraction does not establish that the archive represents every historical message. Proprietary or discontinued systems may require a route that has not yet been established. Begin with the file types, approximate volumes, source history and the evidential question so that a proportionate assessment can be agreed before committing to wider analysis.