IWR1642BOOST: IWR1642 Not working Hwi Interupt of mailbox.

I created custom project for testing Mailbox. I have writen code for MSS and DSS subsystem, and was trying send message from DSS to MSS. Interrupting Mailbox_box_EmptyINT not work, but Interrupting Mailbox_box_FullINT is done. Then I was adding a follow code: 

/*Configuring event combiner*/
    Hwi_Params_init(&params);
    params.enableInt = TRUE;
    for (i = 0; i < 4; i++)
    {   
        params.arg = i;
        params.eventId = i; 
        if (Hwi_create(4 + i, &EventCombiner_dispatch, &params, NULL) == NULL)
        {
            System_printf("failed to create Hwi interrupt %d\n",4 + i);
        }
    }

And sending Mailbox was working well. I have a question.

Why Mailbox sending is not work without that code? Why not work Hardware Interupt for Mailbox_box_EmptyINT without that code?

Help me please)