We should provide minimal information here and then link to the `Action Mailer guide` and have in depth description of the options available. For example:

```
* `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.
```

And then add a description for the possible values in that section.