Int 15 Fn 04  - Multidos Plus - Send Message To Another Task               [T]

   AH = 04h
   AL = mailbox number (00h-3Fh)
   CX = message length in bytes
   DS:SI -> message

Return: AH = status
       00h successful
       01h out of message memory
       02h invalid mailbox number

Note:  the message is copied into a system buffer; the caller may immediately
     reuse its buffer

See Also: AH=05h"MultiDOS"