add_action("wpcf7_before_send_mail", "wpcf7_do_something_else");
function wpcf7_do_something_else(&$wpcf7_data) {
// Here is the variable where the data are stored!
var_dump($wpcf7_data);
// If you want to skip mailing the data, you can do it...
$wpcf7_data->skip_mail = true;
}