Nagios send mail when server is down

I am using nagios 3.06 to monitor the servers. When a service is critical, it sends a mail, but when a server is down no mail is sent. Even if all the services go to critical state, no mail is sent.

I have the following configuration:

define command {
    command_name notify-host-by-email
    command_line python /etc/nagios3/send_mail.py "[Nagios] $HOSTNAME$" "******** Nagios ****\n\n Host: $HOSTNAME$\n Description: the server is down"
}

define command{
    command_name notify-service-by-email
    command_line python /etc/nagios3/send_mail.py "[Nagios] $HOSTNAME$: $SERVICEDESC$ ($NOTIFICATIONTYPE$)" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\nDate/Time: $LONGDATETIME$\nAdditional Info:$SERVICEOUTPUT$"
}

The python script is a script to sent a mail. It works if I execute it from the command line, but it doesn't sents an email from nagios.

What I am doing wrong?

UPDATE: The contact data is:

define contact{
    contact_name                    root
    alias                           Root
    service_notification_period     24x7
    host_notification_period        24x7
    service_notification_options    w,u,c,r
    host_notification_options       d,r
    service_notification_commands   notify-service-by-email
    host_notification_commands      notify-host-by-email
    email                           myemail@gmail.com
}

define contactgroup{
    contactgroup_name       admins
    alias                   Nagios Administrators
    members                 root
}