def is_valid(self, domain, diagnose=False):

        """Check whether a domain has a valid MX or A record.

        Keyword arguments:
        domain   --- the domain to check
        diagnose --- flag to report a diagnosis or a boolean (default False)

        """

        return_status = [ValidDiagnosis()]
        dns_checked = False

        # http://tools.ietf.org/html/rfc5321#section-2.3.5
        #   Names that can be resolved to MX RRs or address (i.e., A or AAAA)
        #   RRs (as discussed in Section 5) are permitted, as are CNAME RRs
        #   whose targets can be resolved, in turn, to MX or address RRs.
        #
        # http://tools.ietf.org/html/rfc5321#section-5.1
        #   The lookup first attempts to locate an MX record associated with
        #   the name.  If a CNAME record is found, the resulting name is
        #   processed as if it were the initial name. ... If an empty list of
        #   MXs is returned, the address is treated as if it was associated
        #   with an implicit MX RR, with a preference of 0, pointing to that
        #   host.
        #
        # is_email() author's note: We will regard the existence of a CNAME to
        # be sufficient evidence of the domain's existence. For performance
        # reasons we will not repeat the DNS lookup for the CNAME's target, but
        # we will raise a warning because we didn't immediately find an MX
        # record.
        try:
            dns.resolver.query(domain, 'MX')
            dns_checked = True
        except (dns.resolver.NXDOMAIN, dns.name.NameTooLong):
            # Domain can't be found in DNS
            return_status.append(DNSDiagnosis('NO_RECORD'))

            # Since dns.resolver gives more information than the PHP analog, we
            # can say that TLDs that throw an NXDOMAIN or NameTooLong error
            # have been checked
            if len(domain.split('.')) == 1:
                dns_checked = True
        except dns.resolver.NoAnswer:
            # MX-record for domain can't be found
            return_status.append(DNSDiagnosis('NO_MX_RECORD'))

            try:
                # TODO: See if we can/need to narrow to A / CNAME
                dns.resolver.query(domain)
            except dns.resolver.NoAnswer:
                # No usable records for the domain can be found
                return_status.append(DNSDiagnosis('NO_RECORD'))
        except dns.resolver.NoNameservers:
            return_status.append(DNSDiagnosis('NO_NAMESERVERS'))
        except (dns.exception.Timeout, dns.resolver.Timeout):
            return_status.append(DNSDiagnosis('DNS_TIMEDOUT'))

        # Check for TLD addresses
        # -----------------------
        # TLD addresses are specifically allowed in RFC 5321 but they are
        # unusual to say the least. We will allocate a separate
        # status to these addresses on the basis that they are more likely
        # to be typos than genuine addresses (unless we've already
        # established that the domain does have an MX record)
        #
        # http://tools.ietf.org/html/rfc5321#section-2.3.5
        #   In the case of a top-level domain used by itself in an address, a
        #   single string is used without any dots. This makes the requirement,
        #   described in more detail below, that only fully-qualified domain
        #   names appear in SMTP transactions on the public Internet,
        #   particularly important where top-level domains are involved.
        #
        # TLD format
        # ----------
        # The format of TLDs has changed a number of times. The standards
        # used by IANA have been largely ignored by ICANN, leading to
        # confusion over the standards being followed. These are not defined
        # anywhere, except as a general component of a DNS host name (a label).
        # However, this could potentially lead to 123.123.123.123 being a
        # valid DNS name (rather than an IP address) and thereby creating
        # an ambiguity. The most authoritative statement on TLD formats that
        # the author can find is in a (rejected!) erratum to RFC 1123
        # submitted by John Klensin, the author of RFC 5321:
        #
        # http://www.rfc-editor.org/errata_search.php?rfc=1123&eid=1353
        #   However, a valid host name can never have the dotted-decimal
        #   form #.#.#.#, since this change does not permit the highest-level
        #   component label to start with a digit even if it is not
        #   all-numeric.
        if not dns_checked:
            atom_list = domain.split(".")
            if len(atom_list) == 1:
                return_status.append(RFC5321Diagnosis('TLD'))

            try:
                float(atom_list[len(atom_list)-1][0])
                return_status.append(RFC5321Diagnosis('TLDNUMERIC'))
            except ValueError:
                pass

        final_status = max(return_status)

        return final_status if diagnose else final_status == ValidDiagnosis()
Check whether a domain has a valid MX or A record.

        Keyword arguments:
        domain   --- the domain to check
        diagnose --- flag to report a diagnosis or a boolean (default False)