activeadmin_devise_mailers.rb

ActiveAdmin.register_page "Mailers" do
  sidebar :mailers do
    ul do
      li do
        link_to "Password Reset", "/rails/mailers/devise_mailer/reset_password_instructions", target: "preview"
      end
      li do
        link_to "Confirmation", "/rails/mailers/devise_mailer/confirmation_instructions", target: "preview"
      end
      li do
        link_to "Unlock", "/rails/mailers/devise_mailer/unlock_instructions", target: "preview"
      end
    end
  end

  content do
    content_tag :iframe, nil, name: "preview", width: "75%", height: "550", seamless: "seamless"
  end
end

users_preview.rb

# Preview all emails at http://localhost:3000/rails/mailers
class DeviseMailerPreview < ActionMailer::Preview
  def confirmation_instructions
    Devise::Mailer.confirmation_instructions(user, "12345678")
  end

  def reset_password_instructions
    Devise::Mailer.reset_password_instructions(user, "12345678")
  end

  def unlock_instructions
    Devise::Mailer.unlock_instructions(user, "12345678")
  end
end