Azure App Service Authentication and Azure AD domain hints
When you're creating web, mobile, API, or Function apps for use by members of your organization, it’s often the case that you’re using Azure Active Directory (Azure AD) and you want to remove the option to sign in with non-organizational credentials. For example, you might want to prevent users from accidentally signing in with Microsoft account credentials (hotmail.com, live.com, outlook.com, etc.). You can do this by leveraging what’s known as a domain hint when you're navigating users to the Azure AD sign-in page.
For more information, see https://blogs.msdn.microsoft.com/appserviceteam/2016/07/13/app-service-auth-and-aad-domain-hints/.