How do I validate members of an array field?

I have this model:

class Campaign

  include Mongoid::Document
  include Mongoid::Timestamps

  field :name, :type => String
  field :subdomain, :type => String
  field :intro, :type => String
  field :body, :type => String
  field :emails, :type => Array
end

Now I want to validate that each email in the emails array is formatted correctly. I read the Mongoid and ActiveModel::Validations documentation but I didn't find how to do this.

Can you show me a pointer?