diff --git a/certbot-postfix/certbot_postfix/installer.py b/certbot-postfix/certbot_postfix/installer.py
index da573baaa..96fc1b4d1 100644
--- a/certbot-postfix/certbot_postfix/installer.py
+++ b/certbot-postfix/certbot_postfix/installer.py
@@ -26,7 +26,6 @@ class Installer(plugins_common.Plugin):
                  policy_config,
                  postfix_dir,
                  fixup=False,
-                 fopen=open,
                  version=None):
         self.fixup          = fixup
         self.postfix_dir    = postfix_dir
@@ -38,7 +37,7 @@ class Installer(plugins_common.Plugin):
         self.additions = []
         self.deletions = []
         self.fn = self.find_postfix_cf()
-        self.raw_cf = fopen(self.fn).readlines()
+        self.raw_cf = open(self.fn).readlines()
         self.cf = map(string.strip, self.raw_cf)
         #self.cf = [line for line in cf if line and not line.startswith("#")]
         self.policy_lines = []
@@ -114,7 +113,7 @@ class Installer(plugins_common.Plugin):
 	self.ensure_cf_var("smtp_tls_protocols", "!SSLv2, !SSLv3", [])
 	self.ensure_cf_var("smtp_tls_mandatory_protocols", "!SSLv2, !SSLv3", [])
 
-    def maybe_add_config_lines(self, fopen=open):
+    def maybe_add_config_lines(self):
         if not self.additions:
             return
         if self.fixup:
@@ -135,10 +134,10 @@ class Installer(plugins_common.Plugin):
                 self.new_cf += line
         self.new_cf += sep + new_cf_lines
 
-        with fopen(self.fn, "w") as f:
+        with open(self.fn, "w") as f:
             f.write(self.new_cf)
 
-    def set_domainwise_tls_policies(self, fopen=open):
+    def set_domainwise_tls_policies(self):
         all_acceptable_mxs = self.policy_config.acceptable_mxs
         for address_domain, properties in all_acceptable_mxs.items():
             mx_list = properties.accept_mx_domains
@@ -164,7 +163,7 @@ class Installer(plugins_common.Plugin):
                 )
             self.policy_lines.append(entry)
 
-        with fopen(self.policy_file, "w") as f:
+        with open(self.policy_file, "w") as f:
             f.write("\n".join(self.policy_lines) + "\n")
 
     ### Let's Encrypt client IPlugin ###
diff --git a/certbot-postfix/certbot_postfix/installer_test.py b/certbot-postfix/certbot_postfix/installer_test.py
index 7ee40a955..93aa171a9 100644
--- a/certbot-postfix/certbot_postfix/installer_test.py
+++ b/certbot-postfix/certbot_postfix/installer_test.py
@@ -6,10 +6,12 @@ from __future__ import division
 from __future__ import print_function
 from __future__ import unicode_literals
 
-import io
 import logging
 import unittest
 
+import mock
+import six
+
 from certbot_postfix import installer
 
 
@@ -28,61 +30,44 @@ certs_only_config = (
 smtpd_tls_key_file = /etc/letsencrypt/live/www.fubard.org/privkey.pem""")
 
 
-def GetFakeOpen(fake_file_contents):
-    fake_file = io.StringIO()
-    # cast this to unicode for py2
-    fake_file.write(fake_file_contents)
-    fake_file.seek(0)
-
-    def FakeOpen(_):
-        return fake_file
-
-    return FakeOpen
-
-  
 class TestPostfixConfigGenerator(unittest.TestCase):
 
     def setUp(self):
-        self.fopen_names_only_config = GetFakeOpen(names_only_config)        
-        self.fopen_certs_only_config = GetFakeOpen(certs_only_config)
-        self.fopen_no_certs_only_config = self.fopen_names_only_config
-
-        #self.config = Config.Config()
         self.config = None
         self.postfix_dir = 'tests/'
 
-    def tearDown(self):
-        pass
-
     def testGetAllNames(self):
         sorted_names = ['fubard.org', 'mail.fubard.org']
-        postfix_config_gen = installer.Installer(
-            self.config,
-            self.postfix_dir,
-            fixup=True,
-            fopen=self.fopen_names_only_config
-        )
+        with mock.patch('certbot_postfix.installer.open') as mock_open:
+            mock_open.return_value = six.StringIO(names_only_config)
+            postfix_config_gen = installer.Installer(
+                self.config,
+                self.postfix_dir,
+                fixup=True,
+            )
         self.assertEqual(sorted_names, postfix_config_gen.get_all_names())
 
     def testGetAllCertAndKeys(self):
         return_vals = [('/etc/letsencrypt/live/www.fubard.org/fullchain.pem',
                         '/etc/letsencrypt/live/www.fubard.org/privkey.pem',
                         'tests/main.cf'),]
-        postfix_config_gen = installer.Installer(
-            self.config,
-            self.postfix_dir,
-            fixup=True,
-            fopen=self.fopen_certs_only_config
-        )
+        with mock.patch('certbot_postfix.installer.open') as mock_open:
+            mock_open.return_value = six.StringIO(certs_only_config)
+            postfix_config_gen = installer.Installer(
+                self.config,
+                self.postfix_dir,
+                fixup=True,
+            )
         self.assertEqual(return_vals, postfix_config_gen.get_all_certs_keys())
 
     def testGetAllCertsAndKeys_With_None(self):
-        postfix_config_gen = installer.Installer(
-            self.config,
-            self.postfix_dir,
-            fixup=True,
-            fopen=self.fopen_no_certs_only_config
-        )
+        with mock.patch('certbot_postfix.installer.open') as mock_open:
+            mock_open.return_value = six.StringIO(names_only_config)
+            postfix_config_gen = installer.Installer(
+                self.config,
+                self.postfix_dir,
+                fixup=True,
+            )
         self.assertEqual([], postfix_config_gen.get_all_certs_keys())