INT 15 - TopView - SEND MESSAGE - "SUBFROM" - SEND MAILBOX MESSAGE/STAT BY REF
        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)