RTOS osMessagePut usage problem

It takes me several days for debbuging ,but didn't work.

when the para 'timeout' of osMessagePut(id,info,timeout) is not set to 0,
then the program crashes with info 'OS_ERROR_MBX_OVF' , which means mailbox overflow.
But with timeout setting to 0,everything seems ok.

It really confuses me. Can anyone help me ? Thanks a lot!

The main frame of my program is as follows
(it is from mdk example 'CAN_Ex', basically not changed much )

osPoolDef(rlcan_mpool, 80, CAN_msg);
osMessageQDef(rlcan_msgq_tx, 20, CAN_msg);

/* Thread1, used to send can message */
while(1)
{
        ...
        /* check if  hardware is free for send */
        if (CAN_hw_tx_empty (ctrl) == CAN_OK)
        {
                CAN_hw_wr (ctrl, msg);     /* Send message */
        }
        else
        {
                ptrmsg = osPoolAlloc(rlcan_mpool);

                  /* for timeout is not 0, it crashes after ISR getting about 16 messages */
                result = osMessagePut(rlcan_msgq_tx, ptrmsg, 10);
                if (result != osOK) osPoolFree(rlcan_mpool, ptrmsg);
        }
        osDelay (500);
}


/* ISR, interrupt generates after msg sending succeeds */

void CAN0_ORed_Message_buffer_IRQHandler (void)
{
        ...
        iflag1 = CANc->IFLAG1; //iflag1 will be set if  msg sending succeeds

        while(iflag1)
        {
                iflag1 = 0;

                 /* it crashes after getting about 16 messages */
                 evt = osMessageGet(rlcan_msgq_tx[CTRL0], 0);
                if (evt.status == osEventMessage)
                {
                        CAN_hw_wr (CTRL,evt.value.p);  /* Send message */
                        osPoolFree(rlcan_mpool, evt.value.p);
                }
                iflag1 = CANc->IFLAG1; //check whether iflag1 is set again
        }
}