Non-routable UPNs. When you synchronize your on-premises directory with %BRAND_OFFICE_365_SHORT%, the user principal names ...

Non-routable UPNs. When you synchronize your on-premises directory with %BRAND_OFFICE_365_SHORT%, the user principal names (UPNs) for each user – for example, [email protected] – are synchronized. The UPN must include a publicly routable domain after the @ character. If the UPNs have a suffix that's not routable, such as billw@contoso.local, you need to change the suffix to match your verified %BRAND_OFFICE_365_SHORT% domain. To learn more, see How to prepare a non-routable domain (such as .local domain) for directory synchronization.