code: def __check_msg_for_headers(self, msg, ** email_headers):
          
          all_headers_found = False
          email_headers['Delivered-To'] = email_headers['To']
          email_headers.pop('To')
          all_headers_found = all(k in msg.keys() for k in email_headers)
  
          return all_headers_found
docs: Checks an Email.Message object for the headers in email_headers.
  
          Following are acceptable header names: ['Delivered-To',
              'Received', 'Return-Path', 'Received-SPF',
              'Authentication-Results', 'DKIM-Signature',
              'DomainKey-Signature', 'From', 'To', 'Message-ID',
              'Subject', 'MIME-Version', 'Content-Type', 'Date',
              'X-Sendgrid-EID', 'Sender'].
  
          @Params
          msg - the Email.message object to check
          email_headers - list of headers to check against
          @Returns
          Boolean whether all the headers were found