Sensitivity Label Permissions

Hello,

I have set up sensitivity labels within my company. I have Public, Standard, Confidential and Highly Confidential. When testing with my external email (e.g. Gmail and Yahoo) I am prompted to enter the one-time passcode when opening an email from my test account. But then I tested with an external user who has an Outlook email and he was not prompted to enter the one-time passcode.

"Authenticated Users" is included in Standard, Confidential and Highly Confidential permission control when setting up the labels.

Is this the normal behavior for the one-time passcode only being prompted for Non-Microsoft emails? Can the one-time passcode be prompted for Microsoft (Outlook) domains? Also how can I have multi-factor authenticator apply to my labels for external clients/users?

Any help would be much appreciated.

Thank you!