rails_mailer_alternate_layout.rb

class ExampleMailer < ActionMailer::Base
  default from: "from@example.com"
  # default layout for ExampleMailer
  layout "customer_email"

  # the order ready is sent to the customer and uses the default layout - customer_email
  def order_ready
    @greeting = "Hi"
    mail to: "to@example.org"
  end

  # the ready to ship email is internal only - use the internal layout
  def ready_to_ship
    @greeting = "Hi Hi"

    mail(to: "internal@example.org") do |format|
      format.html { render :layout => "internal_email" }
      format.text { render :layout => "internal_email" }
    end
  end
end