Index: twisted/mail/test/test_mail.py
===================================================================
--- twisted/mail/test/test_mail.py	(revision 17515)
+++ twisted/mail/test/test_mail.py	(working copy)
@@ -102,6 +102,10 @@
         self.assertEquals(d.popitem(), ('key', 'value'))
         self.assertEquals(len(d), 0)
 
+        dcopy = d.copy()
+        self.assertEquals(d.domains, dcopy.domains)
+        self.assertEquals(d.default, dcopy.default)
+
 class BounceTestCase(unittest.TestCase):
     def setUp(self):
         self.domain = mail.mail.BounceDomain()