3.3.5.2 User Account Objects Without UPN

If the user account object does not have the userPrincipalName attribute ([MS-ADA3] section 2.349) set, the KDC SHOULD<46> send a UPN_DNS_INFO structure ([MS-PAC] section 2.10) containing a user principal name (UPN), constructed by concatenating the user name, the "@" symbol, and the DNS name of the domain.