mail_filter.rb

if Rails.env.staging? || Rails.env.development?
  class MailFilter
    def self.delivering_email(message)
      message.to = message.to.reject {|recipient| recipient !~ /@openlistings.com/i }
      if message.to.blank?
        message.perform_deliveries = false
        Rails.logger.debug "Interceptor prevented sending mail #{message.inspect}!"
      end
      message.subject = "[#{Rails.env.upcase}] " + message.subject
    end
  end
  ActionMailer::Base.register_interceptor(MailFilter)
end