PST Export from Third Party Device

Question

Does anyone have experience successfully exporting email data from a device that wasn't used for day-to-day email exchange?

Further info: I am trying to export data (pst file) from an employee's account without having access to the device they use to access their email (physically or remotely). I've logged into their account from my Outlook App, downloaded a PST, but the data does not seem to all be there.

Is it possible to do this or does one need to do the export from the device that has been used to exchange emails? I get some data when doing the export from my device but I do not seem to get everything.

Any info is appreciated!