Again, as stated in my very first comment https://github.com/rails/rails/pull/13728#discussion_r9005425 we should mention the default:

```
* `config.action_mailer.delivery_method` defines the delivery method and defaults to `:smtp`. 
See the [configuration section in the Action Mailer guide](http://guides.rubyonrails.org/action_mailer_basics.html#action-mailer-configuration) for more info.
```