# File lib/vmail/imap_client.rb, line 621
    def format_headers(hash)
      lines = []
      hash.each_pair do |key, value|
        if value.is_a?(Array)
          value = value.join(", ")
        end
        lines << "#{key.gsub("_", '-')}: #{value}"
      end
      lines.join("\n")
    end