Only way is to enter the FQDN in the OpenVZ as the VM hostname or manually edit /etc/hostname file and to use chattr +i /etc/hostname.
I recommend the first option.
Normally /etc/hostname should not contain FQDN only the host name but this can be problematic on OpenVZ containers and you might have to manually modify on Postfix config's etc. and add the FQDN later.
Last edited by SamTzu; 4th April 2013 at 16:50.