When your on-premises user principal names (UPNs) are synchronized to %BRAND_OFFICE_365_SHORT%, they become the user's login ...

When your on-premises user principal names (UPNs) are synchronized to %BRAND_OFFICE_365_SHORT%, they become the user's login for accessing %BRAND_OFFICE_365_SHORT% services. For the best user experience in %BRAND_OFFICE_365_SHORT%, a user's UPN and primary SMTP email address should be the same. For example, if a user's UPN is [email protected] and his primary email address is [email protected], you'll want to make them match. When these values match, a user only has to remember one set of credentials when accessing %BRAND_OFFICE_365_SHORT% services.