I made the change - here is my code again. I still am only getting out on the first TEST1_CAN_TRANSFER_ID
#include <Wire.h>
#include "variant.h"
#include <due_can.h>
#define TEST1_CAN_COMM_MB_IDX 0
#define TEST1_CAN_TRANSFER_ID 0x07
#define TEST2_CAN_TRANSFER_ID 0x08
#define TEST3_CAN_TRANSFER_ID 0x08
#define TEST4_CAN_TRANSFER_ID 0x09
#define TEST5_CAN_TRANSFER_ID 0x0A
#define TEST1_CAN0_TX_PRIO 15
#define CAN_MSG_DUMMY_DATA 0x55AAAA55
// CAN frame max data length
#define MAX_CAN_FRAME_DATA_LEN 8
// Message variable to be send
uint32_t CAN_MSG_1 = 0;
uint32_t CAN_MSG_2 = 0;
uint32_t CAN_MSG_3 = 0;
uint32_t CAN_MSG_4 = 0;
uint32_t CAN_MSG_5 = 0;
uint32_t CAN_MSG_6 = 0;
uint32_t CAN_MSG_7 = 0;
uint32_t CAN_MSG_8 = 0;
uint32_t CAN_MSG_9 = 0;
uint32_t CAN_MSG_10 = 0;
uint32_t CAN_MSG_11 = 0;
uint32_t CAN_MSG_12 = 0;
// Initialize CAN1 mailbox 0 as receiver, frame ID is 0x07
CAN.init(SystemCoreClock, CAN_BPS_250K);
CAN.reset_all_mailbox();
// Initialize CAN0 mailbox 0 as transmitter, transmit priority is 15
CAN.mailbox_init(0);
CAN.mailbox_init(1);
CAN.mailbox_init(2);
CAN.mailbox_init(3);
CAN.mailbox_init(4);
CAN.mailbox_set_mode(0, CAN_MB_TX_MODE);
CAN.mailbox_set_mode(1, CAN_MB_TX_MODE);
CAN.mailbox_set_mode(2, CAN_MB_TX_MODE);
CAN.mailbox_set_mode(3, CAN_MB_TX_MODE);
CAN.mailbox_set_mode(4, CAN_MB_TX_MODE);
CAN.mailbox_set_priority(0, TEST1_CAN0_TX_PRIO);
CAN.mailbox_set_priority(1, TEST1_CAN0_TX_PRIO);
CAN.mailbox_set_priority(2, TEST1_CAN0_TX_PRIO);
CAN.mailbox_set_priority(3, TEST1_CAN0_TX_PRIO);
CAN.mailbox_set_priority(4, TEST1_CAN0_TX_PRIO);
CAN.mailbox_set_accept_mask(0, 0, false);
// Prepare transmit ID, data and data length in CAN0 mailbox 0
CAN.mailbox_set_id(0, TEST1_CAN_TRANSFER_ID, false);
CAN.mailbox_set_datal(0, CAN_MSG_1);
CAN.mailbox_set_datah(0, CAN_MSG_2);
CAN.mailbox_set_datalen(0, MAX_CAN_FRAME_DATA_LEN);
CAN.mailbox_set_id(1, TEST2_CAN_TRANSFER_ID, false);
CAN.mailbox_set_datal(1, CAN_MSG_3);
CAN.mailbox_set_datah(1, CAN_MSG_DUMMY_DATA);
CAN.mailbox_set_datalen(1, MAX_CAN_FRAME_DATA_LEN);
CAN.mailbox_set_id(2, TEST3_CAN_TRANSFER_ID, false);
CAN.mailbox_set_datal(2, CAN_MSG_5);
CAN.mailbox_set_datah(2, CAN_MSG_6);
CAN.mailbox_set_datalen(2, MAX_CAN_FRAME_DATA_LEN);
CAN.mailbox_set_id(3, TEST4_CAN_TRANSFER_ID, false);
CAN.mailbox_set_datal(3, CAN_MSG_7);
CAN.mailbox_set_datah(3, CAN_MSG_8);
CAN.mailbox_set_datalen(3, MAX_CAN_FRAME_DATA_LEN);
CAN.mailbox_set_id(4, TEST5_CAN_TRANSFER_ID, false);
CAN.mailbox_set_datal(4, CAN_MSG_9);
CAN.mailbox_set_datah(4, CAN_MSG_DUMMY_DATA);
CAN.mailbox_set_datalen(4, MAX_CAN_FRAME_DATA_LEN);
// Send out the information in the mailbox
CAN.global_send_transfer_cmd(CAN_TCR_MB0);
CAN.global_send_transfer_cmd(CAN_TCR_MB1);
CAN.global_send_transfer_cmd(CAN_TCR_MB2);
CAN.global_send_transfer_cmd(CAN_TCR_MB3);
CAN.global_send_transfer_cmd(CAN_TCR_MB4);
CAN.disable();