diff --git a/app/mailers/user_notifications.rb b/app/mailers/user_notifications.rb
index 253e94d9a1c..f71ec0d6856 100644
--- a/app/mailers/user_notifications.rb
+++ b/app/mailers/user_notifications.rb
@@ -84,18 +84,21 @@ class UserNotifications < ActionMailer::Base
   def user_replied(user, opts)
     opts[:allow_reply_by_email] = true
     opts[:use_site_subject] = true
+    opts[:show_category_in_subject] = true
     notification_email(user, opts)
   end
 
   def user_quoted(user, opts)
     opts[:allow_reply_by_email] = true
     opts[:use_site_subject] = true
+    opts[:show_category_in_subject] = true
     notification_email(user, opts)
   end
 
   def user_mentioned(user, opts)
     opts[:allow_reply_by_email] = true
     opts[:use_site_subject] = true
+    opts[:show_category_in_subject] = true
     notification_email(user, opts)
   end
 
@@ -103,6 +106,7 @@ class UserNotifications < ActionMailer::Base
     opts[:allow_reply_by_email] = true
     opts[:use_site_subject] = true
     opts[:add_re_to_subject] = true
+    opts[:show_category_in_subject] = true
     notification_email(user, opts)
   end
 
@@ -110,6 +114,7 @@ class UserNotifications < ActionMailer::Base
     opts[:allow_reply_by_email] = true
     opts[:use_site_subject] = true
     opts[:add_re_to_subject] = true
+    opts[:show_category_in_subject] = false
 
     # We use the 'user_posted' event when you are emailed a post in a PM.
     opts[:notification_type] = 'posted'
@@ -125,6 +130,7 @@ class UserNotifications < ActionMailer::Base
       allow_reply_by_email: true,
       use_site_subject: true,
       add_re_to_subject: true,
+      show_category_in_subject: true,
       notification_type: "posted",
       user: user
     )
@@ -174,6 +180,7 @@ class UserNotifications < ActionMailer::Base
     allow_reply_by_email = opts[:allow_reply_by_email] unless user.suspended?
     use_site_subject = opts[:use_site_subject]
     add_re_to_subject = opts[:add_re_to_subject]
+    show_category_in_subject = opts[:show_category_in_subject]
 
     send_notification_email(
       title: title,
@@ -182,6 +189,7 @@ class UserNotifications < ActionMailer::Base
       allow_reply_by_email: allow_reply_by_email,
       use_site_subject: use_site_subject,
       add_re_to_subject: add_re_to_subject,
+      show_category_in_subject: show_category_in_subject,
       notification_type: notification_type,
       user: user
     )
@@ -198,6 +206,19 @@ class UserNotifications < ActionMailer::Base
     notification_type = opts[:notification_type]
     user = opts[:user]
 
+    # category name
+    category = Topic.find_by(id: post.topic_id).category
+    if opts[:show_category_in_subject] && post.topic_id && !category.uncategorized?
+      show_category_in_subject = category.name
+
+      # subcategory case
+      if !category.parent_category_id.nil?
+        show_category_in_subject = "#{Category.find_by(id: category.parent_category_id).name}/#{show_category_in_subject}"
+      end
+    else
+      show_category_in_subject = nil
+    end
+
     context = ""
     tu = TopicUser.get(post.topic_id, user)
     context_posts = self.class.get_context_posts(post, tu)
@@ -241,6 +262,7 @@ class UserNotifications < ActionMailer::Base
       allow_reply_by_email: allow_reply_by_email,
       use_site_subject: use_site_subject,
       add_re_to_subject: add_re_to_subject,
+      show_category_in_subject: show_category_in_subject,
       private_reply: post.topic.private_message?,
       include_respond_instructions: !user.suspended?,
       template: template,
diff --git a/config/site_settings.yml b/config/site_settings.yml
index b796b292045..a33529226da 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -390,7 +390,7 @@ email:
     default: 7
     enum: 'DigestEmailSiteSetting'
   email_custom_headers: 'Auto-Submitted: auto-generated'
-  email_subject: '%{optional_re}[%{site_name}] %{optional_pm}%{topic_title}'
+  email_subject: '%{optional_re}[%{site_name}] %{optional_pm}%{optional_cat}%{topic_title}'
   reply_by_email_enabled: false
   reply_by_email_address: ''
   pop3_polling_enabled: false
diff --git a/lib/email/message_builder.rb b/lib/email/message_builder.rb
index ed8ea0b7567..cff0e2b640d 100644
--- a/lib/email/message_builder.rb
+++ b/lib/email/message_builder.rb
@@ -45,6 +45,7 @@ module Email
         subject.gsub!("%{site_name}", @template_args[:site_name])
         subject.gsub!("%{optional_re}", @opts[:add_re_to_subject] ? I18n.t('subject_re', template_args) : '')
         subject.gsub!("%{optional_pm}", @opts[:private_reply] ? I18n.t('subject_pm', template_args) : '')
+        subject.gsub!("%{optional_cat}", @template_args[:show_category_in_subject] ? "[#{@template_args[:show_category_in_subject]}] " : '')
         subject.gsub!("%{topic_title}", @template_args[:topic_title]) if @template_args[:topic_title] # must be last for safety
       else
         subject = @opts[:subject]
diff --git a/spec/mailers/user_notifications_spec.rb b/spec/mailers/user_notifications_spec.rb
index 0f612a87403..5ba354ea3fb 100644
--- a/spec/mailers/user_notifications_spec.rb
+++ b/spec/mailers/user_notifications_spec.rb
@@ -71,15 +71,19 @@ describe UserNotifications do
   end
 
   describe '.user_replied' do
-    let(:post) { Fabricate(:post) }
+    let(:category) { Fabricate(:category, name: 'India') }
+    let(:topic) { Fabricate(:topic, category: category) }
+    let(:post) { Fabricate(:post, topic: topic) }
     let(:response) { Fabricate(:post, topic: post.topic)}
     let(:user) { Fabricate(:user) }
     let(:notification) { Fabricate(:notification, user: user) }
 
-
     it 'generates a correct email' do
       mail = UserNotifications.user_replied(response.user, post: response, notification: notification)
 
+      # subject should include category name
+      expect(mail.subject).to match(/India/)
+
       # 2 respond to links cause we have 1 context post
       mail.html_part.to_s.scan(/To respond/).count.should == 2
 
@@ -113,6 +117,9 @@ describe UserNotifications do
     it 'generates a correct email' do
       mail = UserNotifications.user_posted(response.user, post: response, notification: notification)
 
+      # subject should not include category name
+      expect(mail.subject).not_to match(/Uncategorized/)
+
       # subject should include "Re:"
       expect(mail.subject).to match("Re:")
 
@@ -262,13 +269,6 @@ describe UserNotifications do
     end
   end
 
-  describe "user posted" do
-    include_examples "notification email building" do
-      let(:notification_type) { :posted }
-      include_examples "supports reply by email"
-    end
-  end
-
   describe "user invited to a private message" do
     include_examples "notification email building" do
       let(:notification_type) { :invited_to_private_message }