code: def fqdn(self):
          
          if self._fqdn is None:
              # Let's try to retrieve it:
              self._fqdn = socket.getfqdn()
  
              if "." not in self._fqdn:
                  try:
                      info = socket.getaddrinfo(
                          host="localhost", port=None, proto=socket.IPPROTO_TCP
                      )
                  except socket.gaierror:
                      addr = "127.0.0.1"
                  else:
                      # We only consider the first returned result and we're
                      # only interested in getting the IP(v4 or v6) address:
                      addr = info[0][4][0]
  
                  self._fqdn = "[{}]".format(addr)
  
          return self._fqdn
docs: Returns the string used to identify the client when initiating a SMTP
          session.
  
          RFC 5321 `§ 4.1.1.1`_ and `§ 4.1.3`_ tell us what to do:
  
          - Use the client FQDN ;
          - If it isn't available, we SHOULD fall back to an address literal.
  
          Returns:
              str: The value that should be used as the client FQDN.
  
          .. _`§ 4.1.1.1`: https://tools.ietf.org/html/rfc5321#section-4.1.1.1
          .. _`§ 4.1.3`: https//tools.ietf.org/html/rfc5321#section-4.1.3