diff --git a/lib/mailtrap/action_mailer/delivery_method.rb b/lib/mailtrap/action_mailer/delivery_method.rb
index 6fa3bd5..f0c0929 100644
--- a/lib/mailtrap/action_mailer/delivery_method.rb
+++ b/lib/mailtrap/action_mailer/delivery_method.rb
@@ -14,7 +14,9 @@ def initialize(settings)
       def deliver!(message)
         mail = Mailtrap::Mail.from_message(message)
 
-        client.send(mail)
+        client.send(mail).tap do |resp|
+          message['X-MT-Message-IDs'] = resp[:message_ids]
+        end
       end
 
       private
diff --git a/spec/mailtrap/action_mailer/delivery_method_spec.rb b/spec/mailtrap/action_mailer/delivery_method_spec.rb
index a88784f..f444667 100644
--- a/spec/mailtrap/action_mailer/delivery_method_spec.rb
+++ b/spec/mailtrap/action_mailer/delivery_method_spec.rb
@@ -48,6 +48,7 @@
 
     it 'converts the message and sends via API' do
       expect(deliver!).to eq({ success: true, message_ids: expected_message_ids })
+      expect(message['X-MT-Message-IDs'].to_s).to eq(expected_message_ids.join(', '))
       expect(Mailtrap::Client).to have_received(:new).with(api_key: 'correct-api-key')
     end
   end