diff --git a/Gemfile.lock b/Gemfile.lock
new file mode 100644
index 0000000..5ebd8cf
--- /dev/null
+++ b/Gemfile.lock
@@ -0,0 +1,59 @@
+PATH
+  remote: .
+  specs:
+    amazon_ses_mailer (0.1.0)
+      aws-sdk-sesv2 (~> 1.0)
+
+GEM
+  remote: https://rubygems.org/
+  specs:
+    aws-eventstream (1.2.0)
+    aws-partitions (1.631.0)
+    aws-sdk-core (3.149.0)
+      aws-eventstream (~> 1, >= 1.0.2)
+      aws-partitions (~> 1, >= 1.525.0)
+      aws-sigv4 (~> 1.1)
+      jmespath (~> 1, >= 1.6.1)
+    aws-sdk-sesv2 (1.27.0)
+      aws-sdk-core (~> 3, >= 3.127.0)
+      aws-sigv4 (~> 1.1)
+    aws-sigv4 (1.5.1)
+      aws-eventstream (~> 1, >= 1.0.2)
+    diff-lcs (1.5.0)
+    docile (1.3.5)
+    jmespath (1.6.1)
+    json (2.6.2)
+    rake (10.5.0)
+    rspec (3.11.0)
+      rspec-core (~> 3.11.0)
+      rspec-expectations (~> 3.11.0)
+      rspec-mocks (~> 3.11.0)
+    rspec-core (3.11.0)
+      rspec-support (~> 3.11.0)
+    rspec-expectations (3.11.1)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.11.0)
+    rspec-mocks (3.11.1)
+      diff-lcs (>= 1.2.0, < 2.0)
+      rspec-support (~> 3.11.0)
+    rspec-support (3.11.1)
+    simplecov (0.17.1)
+      docile (~> 1.1)
+      json (>= 1.8, < 3)
+      simplecov-html (~> 0.10.0)
+    simplecov-html (0.10.2)
+    simplecov-lcov (0.7.0)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  amazon_ses_mailer!
+  bundler (~> 1.17)
+  rake (~> 10.0)
+  rspec (~> 3.0)
+  simplecov (~> 0.17.1)
+  simplecov-lcov (~> 0.7.0)
+
+BUNDLED WITH
+   1.17.3
diff --git a/amazon_ses_mailer.gemspec b/amazon_ses_mailer.gemspec
index 7182ab2..b7b605f 100644
--- a/amazon_ses_mailer.gemspec
+++ b/amazon_ses_mailer.gemspec
@@ -24,6 +24,8 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 1.17"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.0"
+  spec.add_development_dependency "simplecov", "~> 0.17.1"
+  spec.add_development_dependency "simplecov-lcov", "~> 0.7.0"
 
   spec.add_runtime_dependency "aws-sdk-sesv2", "~> 1.0"
 end
diff --git a/lib/amazon_ses_mailer/base.rb b/lib/amazon_ses_mailer/base.rb
index 0e9c741..e107ac8 100644
--- a/lib/amazon_ses_mailer/base.rb
+++ b/lib/amazon_ses_mailer/base.rb
@@ -1,10 +1,12 @@
+require "./lib/core_extensions/hash.rb"
+
 module AmazonSesMailer
   class Base
     attr_reader :template_name
 
     class << self
       attr_accessor :default_options, :delivery_method
-      
+
       def deliveries
         @@_deliveries ||= []
       end
@@ -57,9 +59,9 @@ def process_merge_vars(merge_vars)
     end
 
     def convert_instance_variables_to_merge_vars
-      self.instance_variable_names.reduce({}) do |result, variable_name|
+      instance_variable_names.reduce({}) do |result, variable_name|
         key = variable_name.delete "@"
-        value = self.instance_variable_get(variable_name)
+        value = instance_variable_get(variable_name)
         result.merge!(key => value)
       end
     end
@@ -67,11 +69,11 @@ def convert_instance_variables_to_merge_vars
     def transform_hash(hash)
       hash.transform_values{ |value| transform_value(value) }
     end
-    
+
     def transform_array(arr)
       arr.map{|value| transform_value(value)}
     end
-    
+
     def transform_value(value)
       # recurse on hashes/arrays, converts nil/false values to empty strings, and converts all others to strings
       return '' unless !!value
@@ -79,5 +81,9 @@ def transform_value(value)
       return transform_array(value) if value.is_a?(Array)
       return value.to_s
     end
+
+    def instance_variable_names
+      instance_variables.map(&:to_s)
+    end
   end
 end
diff --git a/lib/core_extensions/hash.rb b/lib/core_extensions/hash.rb
new file mode 100644
index 0000000..3ef223f
--- /dev/null
+++ b/lib/core_extensions/hash.rb
@@ -0,0 +1,15 @@
+module CoreExtensions
+  module Hash
+    def transform_values
+      return enum_for(:transform_values) { size } unless block_given?
+      return {} if empty?
+      result = self.class.new
+      each do |key, value|
+        result[key] = yield(value)
+      end
+      result
+    end
+  end
+end
+
+Hash.include CoreExtensions::Hash
diff --git a/spec/amazon_ses_mailer/base_spec.rb b/spec/amazon_ses_mailer/base_spec.rb
new file mode 100644
index 0000000..9142b9d
--- /dev/null
+++ b/spec/amazon_ses_mailer/base_spec.rb
@@ -0,0 +1,18 @@
+RSpec.describe AmazonSesMailer::Base do
+  let(:options) { {} }
+  let(:template_name) { '' }
+  let(:delivery_method) { nil }
+  subject { described_class.new(template_name) }
+
+  describe '#mail' do
+    before do
+      allow(AmazonSesMailer::Base).to receieve(:delivery_method)
+        .and_return(delivery_method)
+    end
+
+    it 'creates a new message' do
+      expect(AmazonSesMailer::Message).to receive(:new)
+      subject.mail(options)
+    end
+  end
+end
diff --git a/spec/core_extensions/hash_spec.rb b/spec/core_extensions/hash_spec.rb
new file mode 100644
index 0000000..8fa2810
--- /dev/null
+++ b/spec/core_extensions/hash_spec.rb
@@ -0,0 +1,10 @@
+RSpec.describe CoreExtensions::Hash do
+  describe '#transform_values' do
+    it 'transforms hash values' do
+      original = { a: 'a', b: 'b' }
+      mapped = original.transform_values { |val| "#{val}!" }
+
+      expect(mapped[:a]).to eq 'a!'
+    end
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index d0acb7a..36c1bf4 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,5 +1,23 @@
-require "bundler/setup"
 require "amazon_ses_mailer"
+require 'simplecov'
+require 'simplecov-lcov'
+
+SimpleCov.start :rails do
+  if ENV['CI']
+    SimpleCov::Formatter::LcovFormatter.config do |config|
+      config.report_with_single_file = true
+      config.single_report_path = 'coverage/lcov.info'
+    end
+
+    SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter.new [
+      SimpleCov::Formatter::LcovFormatter
+    ]
+  else
+    SimpleCov.formatter = SimpleCov::Formatter::MultiFormatter.new [
+      SimpleCov::Formatter::HTMLFormatter
+    ]
+  end
+end
 
 RSpec.configure do |config|
   # Enable flags like --only-failures and --next-failure
@@ -10,5 +28,12 @@
 
   config.expect_with :rspec do |c|
     c.syntax = :expect
+    c.include_chain_clauses_in_custom_matcher_descriptions = true
   end
+
+  config.mock_with :rspec do |mocks|
+    mocks.verify_partial_doubles = true
+  end
+
+  config.shared_context_metadata_behavior = :apply_to_host_groups
 end