diff --git a/app/mailers/user_notifications.rb b/app/mailers/user_notifications.rb
index 78bdbd722b2..3aca5a93346 100644
--- a/app/mailers/user_notifications.rb
+++ b/app/mailers/user_notifications.rb
@@ -538,7 +538,7 @@ class UserNotifications < ActionMailer::Base
           .visible_tags(Guardian.new(user))
           .joins(:topic_tags)
           .where("topic_tags.topic_id = ?", post.topic_id)
-          .limit(3)
+          .limit(SiteSetting.max_tags_per_topic)
           .pluck(:name)
 
       show_tags_in_subject = tags.any? ? tags.join(" ") : nil
diff --git a/lib/email/message_builder.rb b/lib/email/message_builder.rb
index 9308705cd64..42aefe600a3 100644
--- a/lib/email/message_builder.rb
+++ b/lib/email/message_builder.rb
@@ -161,6 +161,11 @@ module Email
       result['X-Discourse-Post-Id']  = @opts[:post_id].to_s  if @opts[:post_id]
       result['X-Discourse-Topic-Id'] = @opts[:topic_id].to_s if @opts[:topic_id]
 
+      # at this point these have been filtered by the recipient's guardian for visibility,
+      # see UserNotifications#send_notification_email
+      result['X-Discourse-Tags'] = @template_args[:show_tags_in_subject] if @opts[:show_tags_in_subject]
+      result['X-Discourse-Category'] = @template_args[:show_category_in_subject] if @opts[:show_category_in_subject]
+
       # please, don't send us automatic responses...
       result['X-Auto-Response-Suppress'] = 'All'
 
diff --git a/spec/lib/email/message_builder_spec.rb b/spec/lib/email/message_builder_spec.rb
index 00487992c04..4a9c1993bea 100644
--- a/spec/lib/email/message_builder_spec.rb
+++ b/spec/lib/email/message_builder_spec.rb
@@ -155,6 +155,8 @@ RSpec.describe Email::MessageBuilder do
         body: 'hello world',
         topic_id: 1234,
         post_id: 4567,
+        show_tags_in_subject: "foo bar baz",
+        show_category_in_subject: "random"
         }.merge(additional_opts)
       )
     end
@@ -171,6 +173,14 @@ RSpec.describe Email::MessageBuilder do
       expect(message_with_header_args.header_args['Reply-To']).to eq("\"Discourse\" <#{SiteSetting.notification_email}>")
     end
 
+    it "passes through the topic tags" do
+      expect(message_with_header_args.header_args['X-Discourse-Tags']).to eq('foo bar baz')
+    end
+
+    it "passes through the topic category" do
+      expect(message_with_header_args.header_args['X-Discourse-Category']).to eq('random')
+    end
+
     context "when allow_reply_by_email is enabled " do
       let(:additional_opts) { { allow_reply_by_email: true } }
 
diff --git a/spec/mailers/user_notifications_spec.rb b/spec/mailers/user_notifications_spec.rb
index 63e4350633a..6111bdde8f5 100644
--- a/spec/mailers/user_notifications_spec.rb
+++ b/spec/mailers/user_notifications_spec.rb
@@ -400,6 +400,19 @@ RSpec.describe UserNotifications do
       expect(mail_html.scan(/>bobmarley/).count).to eq(1)
     end
 
+    it "the number of tags shown in subject should match max_tags_per_topic" do
+      SiteSetting.email_subject = "[%{site_name}] %{optional_pm}%{optional_cat}%{optional_tags}%{topic_title}"
+      SiteSetting.max_tags_per_topic = 1
+      mail = UserNotifications.user_replied(
+        user,
+        post: response,
+        notification_type: notification.notification_type,
+        notification_data_hash: notification.data_hash
+      )
+      expect(mail.subject).to match(/Taggo/)
+      expect(mail.subject).not_to match(/Taggie/)
+    end
+
     it "doesn't include details when private_email is enabled" do
       SiteSetting.private_email = true
       mail = UserNotifications.user_replied(