# File lib/vmail/imap_client.rb, line 34
    def message_cache
      @message_cache ||= {}
      size = @message_cache.values.reduce(0) {|sum, x| sum + x[:size]}
      if size > 2_000_000 # TODO make this configurable
        log "Pruning message cache; message cache is consuming #{number_to_human_size size}"
        @message_cache.keys[0, @message_cache.size / 2].each {|k| @message_cache.delete(k)}
      end
      @message_cache
    end