Mail::SpamAssassin::RegistryBoundaries - domain delegation
  rules
  - ($hostname, $domain) = split_domain ($fqdn, $is_ascii)
- Cut a fully-qualified hostname into the hostname part and the domain part,
      splitting at the DNS registry boundary.
    Examples:     "www.foo.com" => ( "www", "foo.com" )
    "www.foo.co.uk" => ( "www", "foo.co.uk" )
    If $is_ascii given and true, skip
        idn_to_ascii() conversion 
- $domain = trim_domain($fqdn, $is_ascii)
- Cut a fully-qualified hostname into the hostname part and the domain part,
      returning just the domain.
    Examples:     "www.foo.com" => "foo.com"
    "www.foo.co.uk" => "foo.co.uk"
    If $is_ascii given and true, skip
        idn_to_ascii() conversion 
- $ok = is_domain_valid($dom, $is_ascii)
- Return 1 if the domain/hostname uses valid known
      TLD, "undef" otherwise.
    If $is_ascii given and true, skip
        idn_to_ascii() conversion. Note that this only checks the TLD validity and nothing else.
        To verify that the complete fqdn is in a valid legal format,
        Util::is_fqdn_valid() can additionally be used.