@James Chan_110,Thanks for posting in Q&A.
For your issue, we can create a condition access policy and app protection policy to achieve your goal.
Here are the steps.
1.Create a condition access policy with the following settings.
- In the Users session, select the user or user group you want to assign.
- In the Target resources session, select Cloud apps, and Select Office 365.
- In the Conditions session, select Client apps and select Browser and Mobile apps and desktop clients under Modern authentication clients.
- In the Grant session, select Require app protection policy and set Enable policy On.
2.Creat an app protection policy and assign it to outlook.
https://learn.microsoft.com/en-us/mem/intune/apps/app-protection-policy-settings-android --Android
https://learn.microsoft.com/en-us/mem/intune/apps/app-protection-policy-settings-windows --Windows
https://learn.microsoft.com/en-us/mem/intune/apps/app-protection-policy-settings-ios --iOS
After that, only Outlook client can access O365.
Hope above information can help you.
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