#10839: add new test file that was omitted from checkin

Lib/test/test_email/test_message.py

+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()