Abortable Action Mailer (no longer needed)

action_mailer_abort.rb

module AbortableMailer
  class UndeliverableMailMessage < Mail::Message
    def self.deliver
      false
    end

    def self.deliver!
      false
    end
  end

  class AbortDeliveryError < StandardError
  end

  class Base < ActionMailer::Base
    def abort_delivery
      raise AbortDeliveryError
    end

    def process(*args)
      begin
        super *args
      rescue AbortDeliveryError
        self.message = AbortableMailer::UndeliverableMailMessage.new
      end
    end
  end
end

notification.rb

class Notification < AbortableMailer::Base
  def user_email(user_id)
    abort_delivery if unsubscribed?(user_id)
  end
end

NOTE: as of Rails 3.2.9 this is no longer necessary. Instead you can use the built in NullMail class:

class Notification < AbortableMailer::Base
  def user_email(user_id)
    return NullMail.new if unsubscribed?(user_id)

    # Send the mail normally
    mail(....) 
  end
end