@Richard Berry Thanks for posting in our Q&A.
For error message, it means that the Outlook app is a protected app. We can't open files from Outlook app via an un-protected app.
For this issue, we appreciate your help to collect some information:
1.Please check if there is any app protection policy of Outlook app deployed.
2.If yes, please check the setting "Send org data to other apps" in this app protection policy.
3.Please try to set the setting "Send org data to other apps" to "All apps" and check if you can use MX Player to open MP3 Files from Outlook app.
If there is anything update, feel free to let us know.
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