code: def sanity_check(self, sender, to, subject, plain=None, html=None, cc=None, bcc=None):
          
          if not plain and not html:
              raise NoContentError()
docs: Sanity check the message.
  
          If we have PLAIN and HTML versions, send a multipart alternative
          MIME message, else send whichever we do have.
  
          If we have neither, raise NoContentError
  
          Arguments:
          - `sender`: str
          - `to`: list
          - `subject`: str
          - `plain`: str
          - `html`: str
  
          Return: None
          Exceptions: NoContentError