In Office 365 Active Directory, the User Principal Name (UPN) is the main ID for the user and doesn't need to match their email address though it typically is their email address. Active Directory also keeps another field for email which may be blank based on the AD implementation. In 365, users login with their UPN identifier rather than the value stored in their email field.
For syncing purposes, CloudRadial relies on the user's UPN as their email address for syncing across systems including synchronization with the PSA contacts. This works fine most of the time, unless Azure AD is synced from another system such as when joined to an on-premise system. In this environment, there be may times when you'd rather sync based on the user's stored 365 email address rather than their UPN.
In CloudRadial, under a company's Office 365 sync settings there is the option to disable the use of the UPN as the primary sync option. When this option is unchecked, the user's email address will be used as the primary key for syncing if available.