diff --git a/Lib/test/mock_socket.py b/Lib/test/mock_socket.py
index 8ef0ec8..e36724f 100644
--- a/Lib/test/mock_socket.py
+++ b/Lib/test/mock_socket.py
@@ -21,8 +21,13 @@
     """
     def __init__(self, lines):
         self.lines = lines
-    def readline(self):
-        return self.lines.pop(0) + b'\r\n'
+    def readline(self, limit=-1):
+        result = self.lines.pop(0) + b'\r\n'
+        if limit >= 0:
+            # Re-insert the line, removing the \r\n we added.
+            self.lines.insert(0, result[limit:-2])
+            result = result[:limit]
+        return result
     def close(self):
         pass
 
diff --git a/Lib/test/test_smtplib.py b/Lib/test/test_smtplib.py
index 39bf7ae..3f7b9b4 100644
--- a/Lib/test/test_smtplib.py
+++ b/Lib/test/test_smtplib.py
@@ -563,6 +563,33 @@
                             HOST, self.port, 'localhost', 3)
 
 
+@unittest.skipUnless(threading, 'Threading required for this test.')
+class TooLongLineTests(unittest.TestCase):
+    respdata = b'250 OK' + (b'.' * smtplib._MAXLINE * 2) + b'\n'
+
+    def setUp(self):
+        self.old_stdout = sys.stdout
+        self.output = io.StringIO()
+        sys.stdout = self.output
+
+        self.evt = threading.Event()
+        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        self.sock.settimeout(15)
+        self.port = support.bind_port(self.sock)
+        servargs = (self.evt, self.respdata, self.sock)
+        threading.Thread(target=server, args=servargs).start()
+        self.evt.wait()
+        self.evt.clear()
+
+    def tearDown(self):
+        self.evt.wait()
+        sys.stdout = self.old_stdout
+
+    def testLineTooLong(self):
+        self.assertRaises(smtplib.SMTPResponseException, smtplib.SMTP,
+                          HOST, self.port, 'localhost', 3)
+
+
 sim_users = {'Mr.A@somewhere.com':'John A',
              'Ms.B@xn--fo-fka.com':'Sally B',
              'Mrs.C@somewhereesle.com':'Ruth C',
@@ -888,7 +915,8 @@
 def test_main(verbose=None):
     support.run_unittest(GeneralTests, DebuggingServerTests,
                               NonConnectingTests,
-                              BadHELOServerTests, SMTPSimTests)
+                              BadHELOServerTests, SMTPSimTests,
+                              TooLongLineTests)
 
 if __name__ == '__main__':
     test_main()