development.rb

# Letter opener config:
# https://github.com/ryanb/letter_opener
  ...
  # Change mail delvery to either :smtp, :sendmail, :file, :test
  config.action_mailer.delivery_method = :letter_opener

....

production.rb

## Mandrill settings:
...
  # ActionMailer Config
  # Setup for production - deliveries, no errors raised
  config.action_mailer.default_url_options = { :host => 'www.example.com' }
  config.action_mailer.delivery_method = :smtp
  config.action_mailer.perform_deliveries = true
  config.action_mailer.raise_delivery_errors = false
  config.action_mailer.default :charset => "utf-8"
  config.action_mailer.smtp_settings = {
    :address        => 'smtp.mandrillapp.com',
    :port           => '587',
    :user_name      => ENV['MANDRILL_USERNAME'],
    :password       => ENV['MANDRILL_API_KEY']
  }
  ...