mailer.rb

class Mailer < ActionMailer::Base
  default :from => "[email protected]"
  helper :formatting

  def new_notification(user)
    @idea = idea
    @user = user
    headers("X-Foo" => "foo")
    mail :to      => user.email,
         :subject => "Subject",
         "X-Bar"  => "foo"
  end
end

sending.rb

email = Mailer.new_notification(user)
p email.headers # => {} WTF??
email.deliver