When you copy a user in Active Directory Users and Computers, the UPN box next to the user name contains corrupted data instead of @DomainName.com.

If the UPN attribute does NOT contain UserName@DomainName.com, you may experience this corruption.

To workaround this issue, change the UPN attribute for the user object to the correct UserName@DomainName.com string by using Ldp.exe or AdsiEdit.exe.