INT 15 - TopView - SEND MESSAGE - "ADDTO" - SEND MAILBOX MESSAGE/STAT BY VALUE
        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)