diff --git a/modoboa/admin/lib.py b/modoboa/admin/lib.py
index 84c7b39bb..ef5c94dc6 100644
--- a/modoboa/admin/lib.py
+++ b/modoboa/admin/lib.py
@@ -175,10 +175,9 @@ def get_domain_mx_list(domain):
     """Return a list of MX IP address for domain."""
     result = []
     logger = logging.getLogger("modoboa.admin")
-    
-    use_specific_dns_server = param_tools.get_global_parameter("use_specific_dns_server")
-    if use_specific_dns_server :
-      dns.resolver.default_resolver.nameservers = [use_specific_dns_server]
+    dns_server = param_tools.get_global_parameter("use_specific_dns_server")
+    if dns_server:
+        dns.resolver.default_resolver.nameservers = [dns_server]
 
     try:
         dns_answers = dns.resolver.query(domain, "MX")
diff --git a/modoboa/admin/tests/test_mx.py b/modoboa/admin/tests/test_mx.py
index fe8bd7d27..70cbca379 100644
--- a/modoboa/admin/tests/test_mx.py
+++ b/modoboa/admin/tests/test_mx.py
@@ -14,7 +14,6 @@ from django.utils.translation import ugettext as _
 
 from modoboa.core import factories as core_factories
 from modoboa.lib.tests import ModoTestCase
-from modoboa.parameters import tools as param_tools
 from . import utils
 from .. import factories, models
 from ..lib import get_domain_mx_list
@@ -108,15 +107,16 @@ class MXTestCase(ModoTestCase):
 
         management.call_command(
             "modo", "check_mx", "--domain", "toto.com")
-        
+
     @mock.patch("socket.getaddrinfo")
     @mock.patch.object(dns.resolver.Resolver, "query")
-    def test_get_domain_mx_list_with_dsn_server(self, mock_query, mock_getaddrinfo):
-      mock_query.side_effect = utils.mock_dns_query_result
-      mock_getaddrinfo.side_effect = utils.mock_ip_query_result
-      self.set_global_parameter("use_specific_dns_server","123.45.67.89")
-      get_domain_mx_list("does-not-exist.example.com")
-    
+    def test_get_domain_mx_list_dsn_server(self, mock_query, mock_getaddrinfo):
+        """Test to get mx list from specific DNS server."""
+        mock_query.side_effect = utils.mock_dns_query_result
+        mock_getaddrinfo.side_effect = utils.mock_ip_query_result
+        self.set_global_parameter("use_specific_dns_server", "123.45.67.89")
+        get_domain_mx_list("does-not-exist.example.com")
+
     @mock.patch("socket.getaddrinfo")
     @mock.patch.object(dns.resolver.Resolver, "query")
     def test_get_domain_mx_list_logging(self, mock_query, mock_getaddrinfo):