How do I back up the X1 archive?

Something happened to Outlook where my main PST file has been lost after a reboot. I have searched the entire computer, checked backups (which unfortunately, only go back for a couple of days) and tried using file recovery software to find the missing PST, but it appears to be gone.

However, X1 still has cached copies of my email. I want to preserve this data. How can I back this up?

Thanks!

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