valid_number?

# File lib/net/imap.rb, line 1616
        def valid_number?(num)
          # [RFC 3501]
          # number          = 1*DIGIT
          #                    ; Unsigned 32-bit integer
          #                    ; (0 <= n < 4,294,967,296)
          num >= 0 && num < 4294967296
        end