Using the New Outlook for Mac features in business environments
Hello Chris Allison,
Regarding your question about whether turning off or hiding the Reading Pane completely prevents email content from being loaded in the background.
Because this concerns the internal behavior of Outlook and potential security implications, I recommend contacting Microsoft Support directly to obtain an authoritative answer from the product team.
You can contact Microsoft Support through New Outlook using the guidance provided here: Contact support and provide feedback in New Outlook for Windows
Also, if you are an admin of the organization, you can submit a support request directly through the Microsoft 365 Admin Center by following the guidance in Get support - Microsoft 365 admin | Microsoft Learn. If you are an end user and do not have administrative access, please contact your IT administrator and ask them to raise a support ticket on your behalf.
If you have any further concern, please feel free to let me know any time.