    # 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