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