Selective authentication for (sub-)domains in Office 365

Office 365 provides various authentication options, such as cloud-IDs, Password Hash Synchronization or federated identities. Leaving out the specifics on how each of these options work, all of them are configured per domain. Whenever trying to access services in Office 365, the user is required to authenticate using its User Principal Name. For sake of simplicity, the general advise it to configure the UPN to match the email address which makes it less confusing for them.

Blog Identity & Security Office 365