diff --git a/tests/test_mx_check.py b/tests/test_mx_check.py
index ebf9119..c84f28d 100644
--- a/tests/test_mx_check.py
+++ b/tests/test_mx_check.py
@@ -1,3 +1,4 @@
+from smtplib import SMTP, SMTPResponseException
 from types import SimpleNamespace
 from unittest.case import TestCase
 from unittest.mock import Mock, patch
@@ -6,8 +7,11 @@ from dns.exception import Timeout
 
 from validate_email import mx_check as mx_module
 from validate_email.email_address import EmailAddress
-from validate_email.exceptions import DNSTimeoutError, NoValidMXError
-from validate_email.mx_check import _get_cleaned_mx_records, mx_check
+from validate_email.exceptions import (
+    DNSTimeoutError, NoValidMXError, SMTPCommunicationError, SMTPMessage,
+    SMTPTemporaryError)
+from validate_email.mx_check import (
+    _get_cleaned_mx_records, _SMTPChecker, mx_check)
 
 
 class DnsNameStub(object):
@@ -66,10 +70,42 @@ class GetMxRecordsTestCase(TestCase):
             _get_cleaned_mx_records(domain='testdomain3', timeout=10)
         self.assertTupleEqual(exc.exception.args, ())
 
-    @patch.object(target=mx_module._SMTPChecker, attribute='check')
+    @patch.object(target=_SMTPChecker, attribute='check')
     def test_skip_smtp_argument(self, check_mx_records_mock):
         'Check correct work of `skip_smtp` argument.'
         self.assertTrue(mx_check(
             EmailAddress('test@mail.ru'), debug=False, skip_smtp=True))
         self.assertEqual(check_mx_records_mock.call_count, 0)
+        check_mx_records_mock.call_count
 
+
+class SMTPCheckerTest(TestCase):
+    'Checking the `_SMTPChecker` class functions.'
+
+    @patch.object(target=SMTP, attribute='connect')
+    def test_connect_raises_serverdisconnected(self, mock_connect):
+        'Connect raises `SMTPServerDisconnected`.'
+        mock_connect.side_effect = OSError('test message')
+        checker = _SMTPChecker(
+            local_hostname='localhost', timeout=5, debug=False,
+            sender='test@example.com', recip='test@example.com')
+        with self.assertRaises(SMTPCommunicationError) as exc:
+            checker.check(hosts=['testhost'])
+        self.assertDictEqual(exc.exception.error_messages, {
+            'testhost': SMTPMessage(
+                command='connect', code=0, text='test message')
+        })
+
+    @patch.object(target=SMTP, attribute='connect')
+    def test_connect_with_error(self, mock_connect):
+        'Connect raises `SMTPServerDisconnected`.'
+        checker = _SMTPChecker(
+            local_hostname='localhost', timeout=5, debug=False,
+            sender='test@example.com', recip='test@example.com')
+        mock_connect.return_value = (400, b'test delay message')
+        with self.assertRaises(SMTPTemporaryError) as exc:
+            checker.check(hosts=['testhost'])
+        self.assertDictEqual(exc.exception.error_messages, {
+            'testhost': SMTPMessage(
+                command='connect', code=400, text='test delay message')
+        })