Resolve the log in issue for a user having a non-existing email id

1) Added CHECK_EMAIL_DELIVERABILITY & SECURITY_EMAIL_VALIDATOR_ARGS.
  2) Added test cases for deliverability check.

Fixes #6550

web/pgadmin/tools/user_management/tests/test_validate_user_email.py

@@ -0,0 +1,62 @@
+##########################################################################
+#
+# pgAdmin 4 - PostgreSQL Tools
+#
+# Copyright (C) 2013 - 2021, The pgAdmin Development Team
+# This software is released under the PostgreSQL Licence
+#
+##########################################################################
+
+from pgadmin.utils.route import BaseTestGenerator
+from pgadmin.tools.user_management import validate_user
+from unittest.mock import patch
+import config
+
+
+class TestValidateUser(BaseTestGenerator):
+    """ This class will test the user email validation with/without email
+    deliverability while validating user. """
+
+    scenarios = [
+        ('User email validation (no deliverability)',
+         dict(
+             data=dict(
+                 email='postgres@local.dev',
+                 check_deliverability=False,
+                 expected_data=dict(
+                     test_result='postgres@local.dev'
+                 )
+             )
+         )),
+        ('User email validation (with deliverability)',
+         dict(
+             data=dict(
+                 email='postgres@local.dev',
+                 check_deliverability=True,
+                 expected_data=dict(
+                     test_result='Invalid email address.'
+                 )
+             )
+         ))
+    ]
+
+    @patch('pgadmin.tools.user_management.validate_password')
+    def runTest(self, validate_password_mock):
+
+        if config.SERVER_MODE is False:
+            self.skipTest(
+                "Can not email validation test cases in the DESKTOP mode."
+            )
+        config.CHECK_EMAIL_DELIVERABILITY = self.data['check_deliverability']
+        ndata = {}
+
+        validate_password_mock.return_value.method.return_value = ''
+        try:
+            ndata = validate_user(self.data)
+        except Exception as e:
+            ndata['email'] = str(e.description)
+
+        # assert equal means deliverability is not done, and entered
+        # email id is returned as it is.
+        self.assertEqual(ndata['email'],
+                         self.data['expected_data']['test_result'])