INT 15 - MultiDOS Plus - SEND MESSAGE TO ANOTHER TASK
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: 1505

1504 - MultiDOS Plus - SEND MESSAGE TO ANOTHER TASK