diff --git a/.circleci/config.yml b/.circleci/config.yml
index 2b8babe..a154bfa 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -3,14 +3,19 @@ version: 2.1
 defaults: &defaults
   working_directory: ~/truemail
   docker:
-    - image: circleci/ruby:2.5.0-node
+    - image: cimg/ruby:<< parameters.ruby-version >>
   environment:
     CC_TEST_REPORTER_ID: 693272a1328521f6f7c09d7ffd419b21c00410da26e98e94c687fdd38b26e2cb
 
 orbs:
-  ruby: circleci/ruby@1.1.2
+  ruby: circleci/ruby@1.1.3
 
 references:
+  install_bundler: &install_bundler
+    run:
+      name: Installing Bundler
+      command: gem i bundler -v $(tail -1 Gemfile.lock | tr -d ' ')
+
   restore_bundle_cache: &restore_bundle_cache
     restore_cache:
       keys:
@@ -29,63 +34,71 @@ references:
 
   install_codeclimate_reporter: &install_codeclimate_reporter
     run:
-      name: Install Code Climate Test Reporter
+      name: Installing CodeClimate test reporter
       command: |
             curl -L https://codeclimate.com/downloads/test-reporter/test-reporter-latest-linux-amd64 > ./cc-test-reporter
             chmod +x ./cc-test-reporter
 
 jobs:
-  linters:
+  linters-ruby:
+    parameters:
+      ruby-version:
+        type: string
+
     <<: *defaults
 
     steps:
       - checkout
 
+      - <<: *install_bundler
       - <<: *restore_bundle_cache
       - <<: *bundle_install
       - <<: *save_bundle_cache
 
       - run:
-          name: Running overcommit
+          name: Running Overcommit
           command: |
             bundle exec overcommit -s
             SKIP=AuthorEmail,AuthorName bundle exec overcommit -r
 
-  tests:
+  tests-ruby:
+    parameters:
+      ruby-version:
+        type: string
+
     <<: *defaults
 
     steps:
       - checkout
 
+      - <<: *install_bundler
       - <<: *restore_bundle_cache
       - <<: *bundle_install
       - <<: *save_bundle_cache
       - <<: *install_codeclimate_reporter
 
       - run:
-          name: Running tests
+          name: Running RSpec
           command: |
-            mkdir /tmp/test-results
             ./cc-test-reporter before-build
             bundle exec rspec
 
       - run:
-          name: Code Climate Test Coverage
+          name: Creating CodeClimate test coverage report
           command: |
             ./cc-test-reporter format-coverage -t simplecov -o "coverage/codeclimate.$CIRCLE_NODE_INDEX.json"
 
-      - store_test_results:
-          path: /tmp/test-results
-
       - store_artifacts:
-          path: /tmp/test-results
-          destination: test-results
+          name: Saving Simplecov coverage artifacts
+          path: ~/truemail/coverage
+          destination: coverage
 
       - deploy:
+          name: Uploading CodeClimate test coverage report
           command: |
             ./cc-test-reporter sum-coverage --output - --parts $CIRCLE_NODE_TOTAL coverage/codeclimate.*.json | ./cc-test-reporter upload-coverage --debug --input -
 
-  compatibility-with-ruby:
+  compatibility-ruby:
     parameters:
       ruby-version:
         type: string
@@ -94,7 +107,6 @@ jobs:
     steps:
       - checkout
       - ruby/install-deps:
-          bundler-version: '1.16.6'
           with-cache: false
           path: './vendor/custom_bundle'
       - run:
@@ -104,9 +116,15 @@ jobs:
 workflows:
   build_and_test:
     jobs:
-      - linters
-      - tests
-      - compatibility-with-ruby:
+      - linters-ruby:
+          matrix:
+            parameters:
+              ruby-version: ["2.5"]
+      - tests-ruby:
+          matrix:
+            parameters:
+              ruby-version: ["2.5"]
+      - compatibility-ruby:
           matrix:
             parameters:
               ruby-version: ["2.6", "2.7", "3.0"]
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 438e69e..97a9da6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,19 @@
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [2.4.2] - 2021.05.13
+
+### Fixed
+
+- Fixed security vulnerability for bundler ([CVE-2019-3881](https://github.com/advisories/GHSA-g98m-96g9-wfjq))
+- Fixed test coverage issues
+
+### Updated
+
+- Updated gem development dependencies
+- Updated simplecov/CircleCi config
+- Updated gem documentation, version
+
 ## [2.4.1] - 2021.05.05
 
 ### Updated
diff --git a/Gemfile.lock b/Gemfile.lock
index 4aee80d..1e4cde5 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    truemail (2.4.1)
+    truemail (2.4.2)
       simpleidn (~> 0.2.1)
 
 GEM
@@ -21,8 +21,8 @@ GEM
     crack (0.4.5)
       rexml
     diff-lcs (1.4.4)
-    dns_mock (1.2.1)
-    docile (1.3.5)
+    dns_mock (1.2.3)
+    docile (1.4.0)
     faker (2.17.0)
       i18n (>= 1.6, < 2)
     fasterer (0.9.0)
@@ -85,7 +85,7 @@ GEM
       unicode-display_width (>= 1.4.0, < 3.0)
     rubocop-ast (1.5.0)
       parser (>= 3.0.1.1)
-    rubocop-performance (1.11.2)
+    rubocop-performance (1.11.3)
       rubocop (>= 1.7.0, < 2.0)
       rubocop-ast (>= 0.4.0)
     rubocop-rspec (2.3.0)
@@ -103,10 +103,10 @@ GEM
     simpleidn (0.2.1)
       unf (~> 0.1.4)
     thor (1.1.0)
-    truemail-rspec (0.5.0)
+    truemail-rspec (0.6.0)
       faker (~> 2.17)
       rspec (~> 3.10)
-      truemail (>= 2.3)
+      truemail (>= 2.4)
     unf (0.1.4)
       unf_ext
     unf_ext (0.0.7.7)
@@ -117,12 +117,12 @@ GEM
       hashdiff (>= 0.4.0, < 2.0.0)
 
 PLATFORMS
-  ruby
+  x86_64-darwin-19
 
 DEPENDENCIES
-  bundler (~> 1.16)
+  bundler (~> 2.2, >= 2.2.17)
   bundler-audit (~> 0.8.0)
-  dns_mock (~> 1.2, >= 1.2.1)
+  dns_mock (~> 1.2, >= 1.2.3)
   faker (~> 2.17)
   fasterer (~> 0.9.0)
   json_matchers (~> 0.11.1)
@@ -132,12 +132,12 @@ DEPENDENCIES
   reek (~> 6.0, >= 6.0.4)
   rspec (~> 3.10)
   rubocop (~> 1.14)
-  rubocop-performance (~> 1.11, >= 1.11.2)
+  rubocop-performance (~> 1.11, >= 1.11.3)
   rubocop-rspec (~> 2.3)
   simplecov (~> 0.17.1)
   truemail!
-  truemail-rspec (~> 0.5)
+  truemail-rspec (~> 0.6)
   webmock (~> 3.12, >= 3.12.2)
 
 BUNDLED WITH
-   1.16.6
+   2.2.17
diff --git a/README.md b/README.md
index 7364540..c0aede1 100644
--- a/README.md
+++ b/README.md
@@ -178,11 +178,12 @@ Truemail.configure do |config|
 
   # Optional parameter. You can predefine default validation type for
   # Truemail.validate('email@email.com') call without with-parameter
-  # Available validation types: :regex, :mx, :smtp
+  # Available validation types: :regex, :mx, :mx_blacklist, :smtp
   config.default_validation_type = :mx
 
   # Optional parameter. You can predefine which type of validation will be used for domains.
-  # Also you can skip validation by domain. Available validation types: :regex, :mx, :smtp
+  # Also you can skip validation by domain.
+  # Available validation types: :regex, :mx, :mx_blacklist, :smtp
   # This configuration will be used over current or default validation type parameter
   # All of validations for 'somedomain.com' will be processed with regex validation only.
   # And all of validations for 'otherdomain.com' will be processed with mx validation only.
@@ -222,7 +223,7 @@ Truemail.configure do |config|
   config.not_rfc_mx_lookup_flow = true
 
   # Optional parameter. This option will provide to use smtp fail fast behaviour. When
-  # smtp_fail_fast = true it means that truemail ends smtp validation session after first
+  # smtp_fail_fast = true it means that Truemail ends smtp validation session after first
   # attempt on the first mx server in any fail cases (network connection/timeout error,
   # smtp validation error). This feature helps to reduce total time of SMTP validation
   # session up to 1 second. By default this option is disabled.
diff --git a/lib/truemail/log/serializer/base.rb b/lib/truemail/log/serializer/base.rb
index 89b1267..ebc1221 100644
--- a/lib/truemail/log/serializer/base.rb
+++ b/lib/truemail/log/serializer/base.rb
@@ -40,20 +40,20 @@ def errors(executor_result_target)
 
         Truemail::Log::Serializer::Base::CONFIGURATION_ARRAY_ATTRS.each do |method|
           define_method(method) do
-            value = executor_configuration.public_send(method)
-            return if value.empty?
-            value
+            executor_configuration_attr = executor_configuration.public_send(method)
+            return if executor_configuration_attr.empty?
+            executor_configuration_attr
           end
         end
 
         Truemail::Log::Serializer::Base::CONFIGURATION_REGEX_ATTRS.each do |method|
           define_method(method) do
-            value = executor_configuration.public_send(method)
+            executor_configuration_attr = executor_configuration.public_send(method)
             default_pattern = Truemail::RegexConstant.const_get(
               (method.eql?(:email_pattern) ? :regex_email_pattern : :regex_smtp_error_body_pattern).upcase
             )
-            return Truemail::Log::Serializer::Base::DEFAULT_GEM_VALUE if value.eql?(default_pattern)
-            value
+            return Truemail::Log::Serializer::Base::DEFAULT_GEM_VALUE if executor_configuration_attr.eql?(default_pattern)
+            executor_configuration_attr
           end
         end
 
diff --git a/lib/truemail/version.rb b/lib/truemail/version.rb
index 8eb2d41..7cca255 100644
--- a/lib/truemail/version.rb
+++ b/lib/truemail/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module Truemail
-  VERSION = '2.4.1'
+  VERSION = '2.4.2'
 end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 5011b5e..9f1af6c 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'bundler/setup'
+require_relative 'support/config/simplecov'
 require_relative '../lib/truemail'
 
 rspec_custom = ::File.join(::File.dirname(__FILE__), 'support/**/*.rb')
diff --git a/spec/support/config/simplecov.rb b/spec/support/config/simplecov.rb
index fb7c231..a78b1e6 100644
--- a/spec/support/config/simplecov.rb
+++ b/spec/support/config/simplecov.rb
@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
-require 'simplecov'
+if ::RUBY_VERSION[/\A2\.5.+\z/]
+  require 'simplecov'
 
-SimpleCov.start
+  SimpleCov.minimum_coverage(100)
+  SimpleCov.start { add_filter(%r{\A/spec/}) }
+end
diff --git a/spec/support/schemas/validator.json b/spec/support/schemas/validator.json
index 7732d87..8f1d922 100644
--- a/spec/support/schemas/validator.json
+++ b/spec/support/schemas/validator.json
@@ -109,6 +109,16 @@
                       "target host(s) not found"
                   ]
               },
+              "mx_blacklist": {
+                "$id": "#/properties/errors/properties/mx_blacklist",
+                "type": "string",
+                "title": "The mx_blacklist schema",
+                "description": "An explanation about the purpose of this instance.",
+                "default": "",
+                "examples": [
+                    "blacklisted mx server ip address"
+                ]
+              },
               "smtp": {
                   "$id": "#/properties/errors/properties/smtp",
                   "type": "string",
diff --git a/spec/truemail/audit/dns_spec.rb b/spec/truemail/audit/dns_spec.rb
index 13e475b..765d3f3 100644
--- a/spec/truemail/audit/dns_spec.rb
+++ b/spec/truemail/audit/dns_spec.rb
@@ -3,7 +3,8 @@
 RSpec.describe Truemail::Audit::Dns do
   let(:configuration_instance) { create_configuration(dns: ["127.0.0.1:#{dns_mock_server.port}"]) }
   let(:verifier_domain) { configuration_instance.verifier_domain }
-  let(:result_instance) { Truemail::Auditor::Result.new(configuration: configuration_instance) }
+  let(:current_host_ip) { random_ip_address }
+  let(:result_instance) { create_auditor(current_host_ip: current_host_ip, configuration: configuration_instance).result }
 
   describe 'defined constants' do
     specify { expect(described_class).to be_const_defined(:VERIFIER_DOMAIN_NOT_REFER) }
@@ -30,12 +31,8 @@
 
     let(:dns_auditor_instance) { described_class.new(result_instance) }
 
-    before { allow(dns_auditor_instance).to receive(:current_host_ip).and_return(current_host_ip) }
-
     describe 'Success' do
       context 'when a record found and refers to current host ip' do
-        let(:current_host_ip) { random_ip_address }
-
         before { dns_mock_server.assign_mocks(verifier_domain => { a: [current_host_ip] }) }
 
         it 'not changes warnings' do
@@ -46,8 +43,6 @@
     end
 
     describe 'Fails' do
-      let(:current_host_ip) { random_ip_address }
-
       shared_examples 'addes verifier domain not refer warning to result instance' do
         it 'addes verifier domain not refer warning to result instance' do
           expect { dns_auditor }
diff --git a/spec/truemail/audit/ip_spec.rb b/spec/truemail/audit/ip_spec.rb
index 9be57eb..7a55e5f 100644
--- a/spec/truemail/audit/ip_spec.rb
+++ b/spec/truemail/audit/ip_spec.rb
@@ -1,8 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe Truemail::Audit::Ip do
-  let(:configuration_instance) { create_configuration }
-  let(:result_instance) { Truemail::Auditor::Result.new(configuration: configuration_instance) }
+  let(:result_instance) { create_auditor(success: nil, warnings: {}).result }
 
   describe 'defined constants' do
     specify { expect(described_class).to be_const_defined(:GET_MY_IP_URL) }
@@ -79,7 +78,7 @@
       end
 
       context 'if network error' do
-        let(:expectation) { expect(ip_auditor_instance).to receive(:detect_ip_via_ipify).and_raise(IPAddr::InvalidAddressError) }
+        let(:expectation) { expect(ip_auditor_instance).to receive(:detect_ip_via_ipify).and_raise(::IPAddr::InvalidAddressError) }
 
         include_examples 'addes ipify warning to result instance'
       end
diff --git a/spec/truemail/audit/ptr_spec.rb b/spec/truemail/audit/ptr_spec.rb
index c345060..b3f36b7 100644
--- a/spec/truemail/audit/ptr_spec.rb
+++ b/spec/truemail/audit/ptr_spec.rb
@@ -3,7 +3,8 @@
 RSpec.describe Truemail::Audit::Ptr do
   let(:configuration_instance) { create_configuration(dns: ["127.0.0.1:#{dns_mock_server.port}"]) }
   let(:verifier_domain) { configuration_instance.verifier_domain }
-  let(:result_instance) { Truemail::Auditor::Result.new(configuration: configuration_instance) }
+  let(:current_host_ip) { random_ip_address }
+  let(:result_instance) { create_auditor(current_host_ip: current_host_ip, configuration: configuration_instance).result }
 
   describe 'defined constants' do
     specify { expect(described_class).to be_const_defined(:PTR_NOT_FOUND) }
@@ -30,9 +31,6 @@
     subject(:ptr_auditor) { ptr_auditor_instance.run }
 
     let(:ptr_auditor_instance) { described_class.new(result_instance) }
-    let(:current_host_ip) { random_ip_address }
-
-    before { allow(ptr_auditor_instance).to receive(:current_host_ip).and_return(current_host_ip) }
 
     describe 'Success' do
       context 'when ptr record exists and refereces to verifier domain' do
diff --git a/spec/truemail/core_spec.rb b/spec/truemail/core_spec.rb
index 3d8a7af..e940a92 100644
--- a/spec/truemail/core_spec.rb
+++ b/spec/truemail/core_spec.rb
@@ -1,13 +1,13 @@
 # frozen_string_literal: true
 
 RSpec.describe Truemail::ConfigurationError do
-  specify { expect(described_class).to be < StandardError }
+  specify { expect(described_class).to be < ::StandardError }
 end
 
 RSpec.describe Truemail::ArgumentError do
   subject(:argument_error_instance) { described_class.new('arg_value', 'arg_name') }
 
-  specify { expect(described_class).to be < StandardError }
+  specify { expect(described_class).to be < ::StandardError }
   specify { expect(argument_error_instance).to be_an_instance_of(described_class) }
   specify { expect(argument_error_instance.to_s).to eq('arg_value is not a valid arg_name') }
 end
diff --git a/spec/truemail/log/serializer/validator_json_spec.rb b/spec/truemail/log/serializer/validator_json_spec.rb
index 768c64b..8ba4d19 100644
--- a/spec/truemail/log/serializer/validator_json_spec.rb
+++ b/spec/truemail/log/serializer/validator_json_spec.rb
@@ -8,21 +8,45 @@
   describe '.call' do
     subject(:json_serializer) { described_class.call(validator_instance) }
 
+    let(:default_configuration_instance) { create_configuration }
+    let(:custom_configuration_instance) do
+      create_configuration(
+        email_pattern: /\A.+@.+\z/,
+        validation_type_for: { random_uniq_domain_name => :regex },
+        whitelisted_domains: [random_uniq_domain_name],
+        blacklisted_domains: [random_uniq_domain_name],
+        blacklisted_mx_ip_addresses: create_servers_list,
+        dns: create_servers_list
+      )
+    end
+
     let(:validator_instance) do
       create_validator(
         validation_type,
         success: success_status,
-        configuration: create_configuration(validation_type_for: { 'somedomain.com' => :regex })
+        configuration: configuration_instance
       )
     end
 
     shared_context 'serialized json' do
-      %i[whitelist regex mx smtp].each do |validation_layer_name|
+      %i[whitelist regex mx mx_blacklist smtp].each do |validation_layer_name|
         describe "#{validation_layer_name} validation" do
           let(:validation_type) { validation_layer_name }
 
-          it 'returns serialized json' do
-            expect(json_serializer).to match_json_schema('validator')
+          context 'with default configuration' do
+            let(:configuration_instance) { default_configuration_instance }
+
+            it 'returns serialized json' do
+              expect(json_serializer).to match_json_schema('validator')
+            end
+          end
+
+          context 'with custom configuration' do
+            let(:configuration_instance) { custom_configuration_instance }
+
+            it 'returns serialized json' do
+              expect(json_serializer).to match_json_schema('validator')
+            end
           end
         end
       end
@@ -53,7 +77,7 @@
         end
 
         it 'replaces all invalid chars' do
-          encoded_utf8_string = JSON.parse(json_serializer)['smtp_debug'].first.dig('errors', 'rcptto')
+          encoded_utf8_string = ::JSON.parse(json_serializer)['smtp_debug'].first.dig('errors', 'rcptto')
           expect(encoded_utf8_string.size).to eq(3)
           expect(encoded_utf8_string).to eq("\uFFFD䲻\uFFFD")
         end
diff --git a/spec/truemail/log/serializer/validator_text_spec.rb b/spec/truemail/log/serializer/validator_text_spec.rb
index 84bee6c..e6cdbaa 100644
--- a/spec/truemail/log/serializer/validator_text_spec.rb
+++ b/spec/truemail/log/serializer/validator_text_spec.rb
@@ -73,7 +73,7 @@
         include_examples 'formatted text output'
       end
 
-      describe 'ip list match validation' do
+      describe 'mx blacklist validation' do
         let(:validation_type) { :mx_blacklist }
 
         include_examples 'formatted text output'
@@ -136,7 +136,7 @@
         include_examples 'formatted text output'
       end
 
-      describe 'ip list match validation' do
+      describe 'mx blacklist validation' do
         let(:configuration_instance) { create_configuration(blacklisted_mx_ip_addresses: mx_servers) }
         let(:validation_type) { :mx_blacklist }
         let(:error) { 'mx blacklist: blacklisted mx server ip address' }
diff --git a/spec/truemail/validate/regex_spec.rb b/spec/truemail/validate/regex_spec.rb
index 14e258a..6862be1 100644
--- a/spec/truemail/validate/regex_spec.rb
+++ b/spec/truemail/validate/regex_spec.rb
@@ -18,23 +18,14 @@
     end
 
     context 'when validation pass' do
-      before do
-        allow(configuration_instance).to receive_message_chain(:email_pattern, :match?).and_return(true)
-      end
-
-      specify do
-        expect { regex_validator }.to change(result_instance, :success).from(nil).to(true)
-      end
+      before { allow(configuration_instance).to receive_message_chain(:email_pattern, :match?).and_return(true) }
 
-      it 'returns true' do
-        expect(regex_validator).to be(true)
-      end
+      specify { expect { regex_validator }.to change(result_instance, :success).from(nil).to(true) }
+      specify { expect(regex_validator).to be(true) }
     end
 
     context 'when validation fails' do
-      before do
-        allow(configuration_instance).to receive_message_chain(:email_pattern, :match?).and_return(false)
-      end
+      before { allow(configuration_instance).to receive_message_chain(:email_pattern, :match?).and_return(false) }
 
       specify do
         expect { regex_validator }
@@ -44,9 +35,7 @@
           .from({}).to(regex: Truemail::Validate::Regex::ERROR)
       end
 
-      it 'returns false' do
-        expect(regex_validator).to be(false)
-      end
+      specify { expect(regex_validator).to be(false) }
     end
   end
 end
diff --git a/spec/truemail/validate/smtp/request_spec.rb b/spec/truemail/validate/smtp/request_spec.rb
index 58f26bf..6f41367 100644
--- a/spec/truemail/validate/smtp/request_spec.rb
+++ b/spec/truemail/validate/smtp/request_spec.rb
@@ -44,7 +44,7 @@
 
     context 'when port opened' do
       specify do
-        allow(Socket).to receive(:tcp)
+        allow(::Socket).to receive(:tcp)
           .with(
             mail_server,
             Truemail::Validate::Smtp::Request::SMTP_PORT,
@@ -58,13 +58,13 @@
 
     context 'when port closed' do
       let(:error_stubs) do
-        allow(Socket).to receive(:tcp)
+        allow(::Socket).to receive(:tcp)
           .with(
             mail_server,
             Truemail::Validate::Smtp::Request::SMTP_PORT,
             connect_timeout: connection_timeout
           )
-          .and_raise(Errno::ETIMEDOUT)
+          .and_raise(::Errno::ETIMEDOUT)
       end
 
       specify do
@@ -81,7 +81,7 @@
       let(:session) { request_instance.send(:session) }
 
       before do
-        allow(Net::SMTP)
+        allow(::Net::SMTP)
           .to receive(:new)
           .with(request_instance_host, Truemail::Validate::Smtp::Request::SMTP_PORT)
           .and_call_original
@@ -103,7 +103,7 @@
     before do
       allow(session).to receive(:open_timeout=).with(connection_timeout)
       allow(session).to receive(:read_timeout=).with(response_timeout)
-      allow(Net::SMTP)
+      allow(::Net::SMTP)
         .to receive(:new)
         .with(request_instance_host, Truemail::Validate::Smtp::Request::SMTP_PORT)
         .and_return(session)
@@ -149,9 +149,9 @@
         )
       end
 
-      context 'when connection timeout error' do
+      context 'when open connection timeout error' do
         let(:error_stubs) do
-          allow(session).to receive(:start).and_raise(Net::OpenTimeout)
+          allow(session).to receive(:start).and_raise(::Net::OpenTimeout)
         end
 
         specify do
@@ -172,10 +172,33 @@
         include_examples 'request retry behavior'
       end
 
+      context 'when read connection timeout error' do
+        let(:error_stubs) do
+          allow(session).to receive(:start).and_raise(::Net::ReadTimeout)
+        end
+
+        specify do
+          error_stubs
+
+          expect { response_instance_target_method }
+            .to change(response_instance, :connection)
+            .from(nil).to(false)
+            .and change(response_instance, :errors)
+            .from({}).to(connection: Truemail::Validate::Smtp::Request::RESPONSE_TIMEOUT_ERROR)
+            .and not_change(response_instance, :helo)
+            .and not_change(response_instance, :mailfrom)
+            .and not_change(response_instance, :rcptto)
+
+          expect(response_instance_target_method).to be(false)
+        end
+
+        include_examples 'request retry behavior'
+      end
+
       context 'when remote server has dropped connection during session' do
         let(:error_stubs) do
-          allow(session).to receive(:start).and_yield(session).and_raise(EOFError)
-          allow(session).to receive(:mailfrom).and_raise(StandardError)
+          allow(session).to receive(:start).and_yield(session).and_raise(::EOFError)
+          allow(session).to receive(:mailfrom).and_raise(::StandardError)
         end
 
         specify do
@@ -200,7 +223,7 @@
 
       context 'when connection other errors' do
         let(:error_stubs) do
-          allow(session).to receive(:start).and_raise(StandardError, error_message)
+          allow(session).to receive(:start).and_raise(::StandardError, error_message)
         end
 
         specify do
@@ -225,7 +248,7 @@
         it 'mailfrom smtp server error' do
           allow(session).to receive(:start).and_yield(session)
           allow(session).to receive(:helo).and_return(true)
-          allow(session).to receive(:mailfrom).and_raise(StandardError, error_message)
+          allow(session).to receive(:mailfrom).and_raise(::StandardError, error_message)
           allow(session).to receive(:rcptto)
 
           expect { response_instance_target_method }
@@ -248,7 +271,7 @@
           allow(session).to receive(:start).and_yield(session)
           allow(session).to receive(:helo).and_return(true)
           allow(session).to receive(:mailfrom).and_return(true)
-          allow(session).to receive(:rcptto).and_raise(StandardError, error_message)
+          allow(session).to receive(:rcptto).and_raise(::StandardError, error_message)
 
           expect { response_instance_target_method }
             .to change(response_instance, :connection)
diff --git a/spec/truemail/wrapper_spec.rb b/spec/truemail/wrapper_spec.rb
index bbfd022..dbd7f62 100644
--- a/spec/truemail/wrapper_spec.rb
+++ b/spec/truemail/wrapper_spec.rb
@@ -38,21 +38,21 @@
     context 'when raises exception' do
       context 'with Resolv::ResolvError exception' do
         specify do
-          allow(mx_instance).to receive(method).and_raise(Resolv::ResolvError)
+          allow(mx_instance).to receive(method).and_raise(::Resolv::ResolvError)
           expect(resolver_execution_wrapper).to be(false)
         end
       end
 
       context 'with IPAddr::InvalidAddressError exception' do
         specify do
-          allow(mx_instance).to receive(method).and_raise(IPAddr::InvalidAddressError)
+          allow(mx_instance).to receive(method).and_raise(::IPAddr::InvalidAddressError)
           expect(resolver_execution_wrapper).to be(false)
         end
       end
 
       context 'with Timeout::Error exception' do
         specify do
-          allow(mx_instance).to receive(method).and_raise(Timeout::Error)
+          allow(mx_instance).to receive(method).and_raise(::Timeout::Error)
 
           expect { resolver_execution_wrapper }
             .to change(resolver_execution_wrapper_instance, :attempts).from(2).to(0)
diff --git a/truemail.gemspec b/truemail.gemspec
index 3e731de..b84f410 100644
--- a/truemail.gemspec
+++ b/truemail.gemspec
@@ -33,9 +33,9 @@ Gem::Specification.new do |spec|
 
   spec.add_runtime_dependency 'simpleidn', '~> 0.2.1'
 
-  spec.add_development_dependency 'bundler', '~> 1.16'
+  spec.add_development_dependency 'bundler', '~> 2.2', '>= 2.2.17'
   spec.add_development_dependency 'bundler-audit', '~> 0.8.0'
-  spec.add_development_dependency 'dns_mock', '~> 1.2', '>= 1.2.1'
+  spec.add_development_dependency 'dns_mock', '~> 1.2', '>= 1.2.3'
   spec.add_development_dependency 'faker', '~> 2.17'
   spec.add_development_dependency 'fasterer', '~> 0.9.0'
   spec.add_development_dependency 'json_matchers', '~> 0.11.1'
@@ -45,9 +45,9 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'reek', '~> 6.0', '>= 6.0.4'
   spec.add_development_dependency 'rspec', '~> 3.10'
   spec.add_development_dependency 'rubocop', '~> 1.14'
-  spec.add_development_dependency 'rubocop-performance', '~> 1.11', '>= 1.11.2'
+  spec.add_development_dependency 'rubocop-performance', '~> 1.11', '>= 1.11.3'
   spec.add_development_dependency 'rubocop-rspec', '~> 2.3'
   spec.add_development_dependency 'simplecov', '~> 0.17.1'
-  spec.add_development_dependency 'truemail-rspec', '~> 0.5'
+  spec.add_development_dependency 'truemail-rspec', '~> 0.6'
   spec.add_development_dependency 'webmock', '~> 3.12', '>= 3.12.2'
 end