INT 15 - MultiDOS Plus - READ MAILBOX
        AH = 06h
        AL = mailbox number (00h-3Fh)
        CX = size of buffer in bytes
        ES:DI -> buffer for message
Return: AH = status
            00h successful
                CX = number of bytes copied
                DX = actual length of message
            02h invalid mailbox number
Note:   if the caller's buffer is not large enough, the message is truncated
          and the remainder is lost
SeeAlso: AH=05h"MultiDOS"