My init code:

   // Enable CAN PIOs
   AT91F_CAN_CfgPIO();
   // Enable CAN Clock
   AT91F_CAN_CfgPMC();

   // Enable CAN Transceiver
   AT91F_PIOA_CfgPMC();

   // Init CAN Interrupt Source Level
   AT91F_AIC_ConfigureIt(AT91C_BASE_AIC,                           // CAN base address
               AT91C_ID_CAN,                            // CAN ID
               AT91C_AIC_PRIOR_HIGHEST,                  // Max priority
               AT91C_AIC_SRCTYPE_INT_HIGH_LEVEL,        // Level sensitive
               AT91F_CAN_Handler);                     // C Handler

   AT91F_AIC_EnableIt(AT91C_BASE_AIC,AT91C_ID_CAN);

   // Cfg CAN Baudrate to 1Mbit/s => BRP = 5
   // We choose 8 Time Quanta (tCSC = 125ns): 8tCSC = tSYNC + tPRS + tPHS1 + tPHS2
   // Cfg PHASE1 PHASE2 PROPAG and SYNC segment
   // Delay of busdriver + delay of receiver circuit = 200ns
   // Delay of bus line considered negligible = 0ns
   // tPRS = 2*(200+0) = 400ns < 4tCSC => PROPAG = 3
   // tSYNC = 1tCSC =>  8tCSC = 1tCSC + 4tCSC + tPHS1 + tPHS2
   // tPHS1 + tPHS2 = 3tCSC => Phase Segment 2 = Max(IPT=2TQ,Phase Segment 1) = 2TQ => PHASE2 = 2-1 = 1 => PHASE1 = 0
   // tSJW = Min(4 TQ, Phase Segment 1) = 1TQ => SJW = 1-1 = 0
   AT91F_CAN_CfgBaudrateReg(AT91C_BASE_CAN,0x00050301);

   enableIRQ(); //enable interrupts

   // Enable CAN and Wait for WakeUp Interrupt
   AT91F_CAN_EnableIt(AT91C_BASE_CAN,AT91C_CAN_WAKEUP);
   AT91F_CAN_CfgModeReg(AT91C_BASE_CAN,AT91C_CAN_CANEN);
   status = (AT91F_CAN_GetStatus(AT91C_BASE_CAN)&ERROR_MASK)>>16; //error active flag set in can status register! Not OK!
   
   // Wait for WAKEUP flag raising <=> 11-recessive-bit were scanned by the transceiver
   while( (testCAN != AT91C_TEST_OK) ) //the wake-up flag is set as expected in the CAN interrupt handler
   {    
   }

// Configure Mailboxes
  CANInitMailboxes();
 
  //End of init.

#ifdef SENDER
    // Ask Transmission on Mailbox 0
    AT91F_CAN_InitTransferRequest(AT91C_BASE_CAN,AT91C_CAN_MB0);
    // Enable Transmission Mailbox 0 interrupt
    AT91F_CAN_EnableIt(AT91C_BASE_CAN,AT91C_CAN_MB0);
#else
    // Enable Reception on Mailbox 1
    AT91F_CAN_InitTransferRequest(AT91C_BASE_CAN,AT91C_CAN_MB1);
    // Enable Mailbox 1 interrupt
    AT91F_CAN_EnableIt(AT91C_BASE_CAN,AT91C_CAN_MB1);
#endif

   /*
   At this point the receiver should be waiting for a message in mailbox 1. The sender should send 1 message via mailbox 0 to the receiver.
   */

   
The CAN interrupt handler:

void AT91F_CAN_Handler(void)
{
  volatile unsigned int status;
      
  status = AT91F_CAN_GetStatus(AT91C_BASE_CAN) & AT91F_CAN_GetInterruptMaskStatus(AT91C_BASE_CAN);
  AT91F_CAN_DisableIt(AT91C_BASE_CAN,status);

  if(status & AT91C_CAN_WAKEUP) { //the wake-up interrupt is triggered, so thats ok.
    testCAN = AT91C_TEST_OK;
    D_TRACE_SIMPLE("=>CAN WAKEUP",1,BLUE);
  }

  if(status & AT91C_CAN_MB0) {
    D_TRACE_SIMPLE("=>CAN MB0 Int",1,BLUE); //this interrupt is not triggered, so message sending fails
  }

  if(status & AT91C_CAN_MB1) {
     D_TRACE_SIMPLE("=>CAN MB1 Int",1,BLUE); //this interrupt is not triggered, so message reception fails
  }
}

My mailbox configuration:

void CANInitMailboxes(void)
{
  // Init CAN Mailbox 0
  //taskENTER_CRITICAL();
  AT91F_InitMailboxRegisters(AT91C_BASE_CAN_MB0,
                             AT91C_CAN_MOT_TX | AT91C_CAN_PRIOR,                    // Mailbox Mode Reg
                             0x00000000,                                            // Mailbox Acceptance Mask Reg
                             AT91C_CAN_MIDvA & (0x07<<18),                           // Mailbox ID Reg
                             0x11223344,                                            // Mailbox Data Low Reg
                             0x01234567,                                            // Mailbox Data High Reg
                             (AT91C_CAN_MDLC & (0x8<<16)) );                        // Mailbox Control Reg
  //taskEXIT_CRITICAL();
  //taskENTER_CRITICAL();
  // Init CANMailbox 1
  AT91F_InitMailboxRegisters(AT91C_BASE_CAN_MB1,
                             AT91C_CAN_MOT_RX | AT91C_CAN_PRIOR,                    // Mailbox Mode Reg
                             AT91C_CAN_MIDvA | AT91C_CAN_MIDvB | AT91C_CAN_MIDE,    // Mailbox Acceptance Mask Reg
                             AT91C_CAN_MIDvA & (0x07<<18),                           // Mailbox ID Reg
                             0x00000000,                                            // Mailbox Data Low Reg
                             0x00000000,                                            // Mailbox Data High Reg
                             0x00000000);                                           // Mailbox Control Reg
  //taskEXIT_CRITICAL();
}