Identify the file and its history
A PST can contain Outlook messages and other items held locally or exported from another source. An OST is an offline data file associated with particular Outlook account arrangements. Identify the client version, account type, computer, collection method and any export or conversion that produced the supplied file.
A cache can be incomplete
Synchronisation settings can limit which material is present locally. The absence of an older message from a cache does not establish that it was absent from the server or never existed. Local changes, synchronisation failures, corruption and later reuse can also affect the evidential picture. New Outlook and classic Outlook have different capabilities that change over time.
Preserve before repair
Keep the original file and work from an appropriately preserved copy. Repair, compaction, importing and conversion can alter the data being assessed; record any necessary processing and retain its output separately. A specialist examination may identify readable items, inconsistencies and possible additional sources, but recoverability depends on the file's condition and history. Access to a local file is not the same as authority to access the live mailbox. Establish both the technical route and the permitted scope before collection.
Sources & further reading
Primary references support the technical explanation. They do not establish D3 validation of a particular method.