sub domain_to_search_list {
  my ($domain) = @_;

  $domain =~ s/^\.+//; $domain =~ s/\.+\z//;      # strip leading and trailing dots
  return [] unless $domain;                       # no domain left
  return [ $domain ] if index($domain, '[') == 0; # don't split address literals

  # initialize
  $domain = lc $domain;
  my @search_keys = ($domain);
  my $pos = 0;

  # split domain into search keys
  while (($pos = index($domain, '.', $pos+1)) != -1) {
    push @search_keys, substr($domain, $pos+1);
  }

  # enforce some sanity limit
  if (@search_keys > 20) {
    @search_keys = @search_keys[$#search_keys-19 .. $#search_keys];
  }

  return \@search_keys;
}