Open Whole Mailbox Utilities
Open Mailbox Utilities from the toolbar or File menu, or press Option-Command-A. Whole-mailbox mode applies the selected action to every message in the PST archive.
Export the archive to MBOX
Choose Export to MBOX and select a destination. PST Office streams messages to a temporary file and replaces the destination only after the export finishes successfully.
Export separate EML files instead
Choose Export All Messages and select a folder to save every convertible message as a separate EML file. PST Office uses safe filenames and reports messages that cannot be converted completely.
Review other complete-archive tools
Whole Mailbox Utilities can also export contacts to VCF, extract readable attachments, and restore the archive to IMAP. Only one whole-mailbox utility runs at a time.