hostname -f says "hostname: the specified hostname is invalid". The hostname (FQDN) specified in /etc/hostname is also listed in /etc/hosts and is pingable. No actual DNS server yet of the host knows. Is this a reason of the error reported or something else? (the OS is Ubuntu 10.04 Server).
Update: registering the host at a DNS server (so that the name can be successfully resolved via Internet) did not help.