rails--rails/actionmailer/test/mailers/proc_mailer.rb

class ProcMailer < ActionMailer::Base
  default to: "system@test.lindsaar.net",
          "X-Proc-Method" => Proc.new { Time.now.to_i.to_s },
          subject: Proc.new { give_a_greeting },
          "x-has-to-proc" => :symbol

  def welcome
    mail
  end

  private

    def give_a_greeting
      "Thanks for signing up this afternoon"
    end
end