diff -r 6619b76053a5 Lib/test/test_email/test_email.py
--- a/Lib/test/test_email/test_email.py	Thu Sep 29 22:31:28 2016 +0200
+++ b/Lib/test/test_email/test_email.py	Mon Oct 10 12:45:46 2016 -0400
@@ -11,6 +11,7 @@
 from io import StringIO, BytesIO
 from itertools import chain
 from random import choice
+from socket import getfqdn
 try:
     from threading import Thread
 except ImportError:
@@ -3314,6 +3315,17 @@
             email.utils.make_msgid(domain='testdomain-string')[-19:],
             '@testdomain-string>')
 
+    def test_make_msgid_idstring(self):
+        self.assertEqual(
+            email.utils.make_msgid(idstring='test-idstring',
+                domain='testdomain-string')[-33:],
+            '.test-idstring@testdomain-string>')
+
+    def test_make_msgid_default_domain(self):
+        self.assertTrue(
+            email.utils.make_msgid().endswith(
+                '@' + getfqdn() + '>'))
+
     def test_Generator_linend(self):
         # Issue 14645.
         with openfile('msg_26.txt', newline='\n') as f: