Checks for bad headers i.e. newlines in subject, sender or recipients.

def is_bad_headers(self):
        """
        Checks for bad headers i.e. newlines in subject, sender or recipients.
        """

        headers = [self.subject, self.sender]
        headers += list(self.send_to)
        headers += dict(self.extra_headers).values()

        for val in headers:
            for c in '\r\n':
                if c in val:
                    return True
        return False