Int 15 Fn 06  - Multidos Plus - Read Mailbox                               [T]

   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

See Also: AH=04h"MultiDOS",AH=05h"MultiDOS"