new_contents: import unittest
  from email import policy
  from test.test_email import TestEmailBase
  
  
  class Test(TestEmailBase):
  
      policy = policy.default
  
      def test_error_on_setitem_if_max_count_exceeded(self):
          m = self._str_msg("")
          m['To'] = 'abc@xyz'
          with self.assertRaises(ValueError):
              m['To'] = 'xyz@abc'
  
  
  if __name__ == '__main__':
      unittest.main()
  
subject: add new test file that was omitted from checkin