When an employee leaves an organization, IT teams usually disable or remove the associated Microsoft 365 or Exchange account. The local OST file may still remain on the employee’s computer or backup storage, containing emails, attachments, calendar entries, contacts, and other mailbox information. The challenge is that the OST was created as a local cache associated with the original account and Outlook profile.

This can create a problem when the organization later needs to review historical mailbox data for an audit, migration, business record, or internal investigation. Recreating the original Outlook environment may not be practical if the account has already been deprovisioned.

A standalone OST viewer can be useful for inspecting the remaining local file. SysInfo OST File Viewer is designed to open OST files without requiring Microsoft Outlook, Exchange, or an active Microsoft 365 profile. It supports single and multiple OST files and can preview mailbox content while maintaining the original folder structure.

The software provides previews for emails, attachments, contacts, calendars, tasks, and notes. It also provides separate tabs for email body, attachments, headers, MIME information, and raw HTML, which can be useful when IT teams need to verify more than the visible email text.

Search and filtering can make the review easier when an old employee mailbox contains a large amount of data. SysInfo OST Viewer allows searches based on From, To, Subject, date range, and attachment availability. Users can then export selected data to formats such as PST, EML, MSG, MBOX, PDF, or CSV.

For offboarding workflows, the important step is to preserve the original OST before making changes to the old computer. The file can then be inspected to determine which information needs to be retained, exported, or moved to another archive.