Int 15 Fn 12  - Topview - Send Message - "addto" - Send Mailbox Message/s  [Q]

   AH = 12h
   BH = 0Ah
   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: the message is copied into either system or common memory
   insufficient memory normally causes the process to be aborted; under
     DESQview 2.2+, failed writes may return CF set instead (see AX=DE15h)

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