ruby-gmail-message.rb

# A snippet from gmail/message.rb

    def label(name)
      @gmail.in_mailbox(@mailbox) do
        begin
          @gmail.imap.uid_copy(uid, name)
        rescue Net::IMAP::NoResponseError
          raise Gmail::NoLabel, "No label `#{name}' exists!"
        end
      end
    end

    def label!(name)
      @gmail.in_mailbox(@mailbox) do
        begin
          @gmail.imap.uid_copy(uid, name)
        rescue Net::IMAP::NoResponseError
          # need to create the label first
          @gmail.create_label(name)
          retry
        end
      end
    end