Authenticating agains AD with Kerberos, by a user with an explicit UPN

Hello

My situation :

I have a 2008 functionnal level domain with a technical name, lets say tec.domain.com

I have for this domain configured an alternate UPN : domain.com (that is only a DNS domain name, not an existing AD domain)

My users have a SamAccountName like j.doe and a UPN like john.doe@domain.com (which is their email address, on our Exchange organization)

Now, from a Linux server (running Apache and kerberos), i can do a kinit with j.doe@TEC.DOMAIN.COM, but not with john.doe@DOMAIN.COM.

When i capture trafic, the DC answers "error-code: eRR-WRONG-REALM (68)", saying it is not able to handle the DOMAIN.COM realm.

According to this article ( http://msdn.microsoft.com/en-us/library/Cc212351.aspx ) , my DC should be able to handle it, as far as i understand it.

Am i missing something ?

Thanks in advance.