capistrano-send/lib/capistrano/tasks/mail.cap

namespace :send do
  namespace :mail do
    desc "Send an email when deploy is finished"
    task :notify do
      if Capistrano::Send::Mail.supported?
        config = fetch(:send_mail, {})

        invoke 'send:set_release_log'

        body = revision_log_message << "\n\n"
        body << "Changes:\n#{fetch(:release_log)}"

        Mail.deliver do
          to config[:to]
          from config[:from]
          subject "Deploy successful on #{fetch(:stage)}!"
          body body

          delivery_method config[:via] || :smtp, config[:via_options] || {}
        end
      end
    end
  end
end

after 'deploy:finished', 'send:mail:notify'