diff --git a/app/mailers/notification_mailers/mentioned_in_comment.rb b/app/mailers/notification_mailers/mentioned_in_comment.rb
index abb346ea8..0e9a51bba 100644
--- a/app/mailers/notification_mailers/mentioned_in_comment.rb
+++ b/app/mailers/notification_mailers/mentioned_in_comment.rb
@@ -5,6 +5,7 @@ module NotificationMailers
     def set_headers(target_id) # rubocop:disable Style/AccessorMethodName
       @comment = Mention.find_by_id(target_id).mentions_container
 
+      @headers[:in_reply_to] = @headers[:references] = "<#{@comment.parent.guid}@#{AppConfig.pod_uri.host}>"
       @headers[:subject] = I18n.t("notifier.mentioned.subject", name: @sender.name)
     end
   end
diff --git a/spec/mailers/notifier_spec.rb b/spec/mailers/notifier_spec.rb
index 2ded0d80f..eb871f7cd 100644
--- a/spec/mailers/notifier_spec.rb
+++ b/spec/mailers/notifier_spec.rb
@@ -120,6 +120,11 @@ describe Notifier, type: :mailer do
       expect(mail.subject).to include(comment.author.name)
     end
 
+    it "IN-REPLY-TO and REFERENCES: references the commented post" do
+      expect(mail.in_reply_to).to eq("#{comment.parent.guid}@#{AppConfig.pod_uri.host}")
+      expect(mail.references).to eq("#{comment.parent.guid}@#{AppConfig.pod_uri.host}")
+    end
+
     it "has the comment link in the body" do
       expect(mail.body.encoded).to include(post_url(comment.parent, anchor: comment.guid))
     end