diff --git a/app/models/email.rb b/app/models/email.rb
index 6e8e8b1..8663f8f 100644
--- a/app/models/email.rb
+++ b/app/models/email.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class Email
   include ActiveModel::Model
   include ActiveModel::Attributes
diff --git a/app/models/metadata_mapping.rb b/app/models/metadata_mapping.rb
index e7afd64..f8be4b9 100644
--- a/app/models/metadata_mapping.rb
+++ b/app/models/metadata_mapping.rb
@@ -1,2 +1,4 @@
+# frozen_string_literal: true
+
 class MetadataMapping < ApplicationRecord
 end
diff --git a/app/models/user.rb b/app/models/user.rb
index ec663a6..ebf0310 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class User < ApplicationRecord
   # Include default devise modules. Others available are:
   # :confirmable, :lockable, :timeoutable, :trackable and :omniauthable
diff --git a/app/services/email_importer.rb b/app/services/email_importer.rb
index 12364c1..2ab9714 100644
--- a/app/services/email_importer.rb
+++ b/app/services/email_importer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class EmailImporter
   attr_accessor :email_class
   attr_accessor :metadata_mapping_class
diff --git a/app/services/html_to_text/base.rb b/app/services/html_to_text/base.rb
index 7d3cc29..e0940cc 100644
--- a/app/services/html_to_text/base.rb
+++ b/app/services/html_to_text/base.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module HtmlToText
   
   class Error < ::StandardError
diff --git a/app/services/html_to_text/elinks.rb b/app/services/html_to_text/elinks.rb
index f0d63aa..4e69b3b 100644
--- a/app/services/html_to_text/elinks.rb
+++ b/app/services/html_to_text/elinks.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module HtmlToText 
   class Elinks < Base
     
diff --git a/app/services/html_to_text/nokogiri.rb b/app/services/html_to_text/nokogiri.rb
index 00ddacb..4003f10 100644
--- a/app/services/html_to_text/nokogiri.rb
+++ b/app/services/html_to_text/nokogiri.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module HtmlToText 
   class Nokogiri < Base