Int 15 Fn 12  - Topview - Send Message - "subfrom" - Send Mailbox Message  [Q]

   AH = 12h
   BH = 0Bh
   BL = mailbox to write to
       00h handle is DWORD on top of stack
       02h default mailbox of task whose handle is on top of stack
       03h current task's default mailbox
   STACK:  DWORD   status (low byte)
       DWORD   length of message
       DWORD   address of message

Notes: only a pointer to the message is stored, but the write may still fail
     due to insufficient memory
   under DV 2.2+, failed mailbox writes may return CF set (see AX=DE15h)

See Also: AH=12h/BH=0Ah"MAILBOX"