Outlook gotcha: only cached data is exported to data file (.PST)

This weekend, a family project that required its own mailbox ended, meaning I could reduce the number of licences in my Exchange Online subscription. That’s straightforward enough but I wanted to take a backup copy of the email before cutting the mailbox loose.

From the last time I did any Exchange Online administration, I recalled that one of the limitations was that you can’t back up a mailbox to a PST from PowerShell. That may have changed but the advice at the time was to backup to an Outlook data file (also known as a Personal Folder) in Outlook. It’s clunky but at least it’s functional.

I couldn’t work out why not all of the data was being exported; only the items that were cached and not the ones that appeared if I clicked on “There are more items in this folder on the server/click here to view more on Microsoft Exchange”. Then I found a clue in a Spiceworks post from Joe Fenninger, where Joe says “Dont [sic] forget to download all [Office 365] content prior to export.”.

I needed to adjust the cached mode settings for the mailbox to change how much email is kept offline, after which Outlook could export all items to the Outlook Data File, rather than just the ones that were cached locally.

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