describe Action Mailer I18n subject lookup

guides/source/i18n.md

@@ -837,6 +837,28 @@ en:
 NOTE: In order to use this helper, you need to install [DynamicForm](https://github.com/joelmoss/dynamic_form)
 gem by adding this line to your Gemfile: `gem 'dynamic_form'`.
 
+### Translations for Action Mailer E-Mail Subjects
+
+If you don't pass a subject to the `mail` method, Action Mailer will try to find
+it in your translations. The performed lookup will use the pattern
+`<mailer_scope>.<action_name>.subject` to construct the key.
+
+```ruby
+# user_mailer.rb
+class UserMailer < ActionMailer::Base
+  def welcome(user)
+    #...
+  end
+end
+```
+
+```yaml
+en:
+  user_mailer:
+    welcome:
+      subject: "Welcome to Rails Guides!"
+```
+
 ### Overview of Other Built-In Methods that Provide I18n Support
 
 Rails uses fixed strings and other localizations, such as format strings and other format information in a couple of helpers. Here's a brief overview.