@Coleman,Darin (CONTR) Thanks for posting in our Q&A.
For this issue, I have done the test in my lab. I deployed Outlook app and add my user under "Available for enrolled devices" assignment. This app is installed successfully in my iOS device.
Please understand that although the Outlook app was deployed through intune, it was actually installed through the Apple app store.
Please try to install Outlook manually via Apple app store. If it failed, there is something wrong with app store or app itself. It is suggested to try to contact Apple to solve this manual installation issue.
Thanks for your understanding.
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