To ensure that users can use single sign-on (SSO) to access Azure resources amid a UPN mismatch, what is the first step you should take?

To enable single sign-on (SSO) for users accessing Azure resources in scenarios where there is a User Principal Name (UPN) mismatch, the most effective first step is to add and verify a custom domain in Azure Active Directory (AD).

When a UPN mismatch occurs, it typically means that the UPN used for a user in on-premises Active Directory does not match the one in Azure AD. By adding a custom domain that aligns with the on-premises UPNs, you establish a connection allowing Azure AD to recognize and authenticate users based on their email addresses associated with the custom domain. This step is crucial, as it sets the foundation for SSO to function correctly, bridging the gap between the on-premises identities and Azure AD.

Once the custom domain is verified in Azure AD, users can then authenticate using their UPN, ensuring seamless access to Azure resources through single sign-on. Without this initial step of domain verification, users would face challenges logging in and accessing services in Azure, even if other actions were taken.

The other options, while potentially relevant in different scopes of configuring SSO or managing identity solutions, do not directly address the immediate requirement of resolving a UPN mismatch through domain verification. Deploying AD FS would