Instead of relying on the user knowing internals of the CAN library - such as the layout of the mailbox groups - use clearly named methods to help the user achieve the desired outcome.

Author: aentinger

libraries/Arduino_CAN/examples/CANReadFilter/CANReadFilter.ino

@@ -35,8 +35,8 @@ void setup()
   Serial.begin(115200);
   while (!Serial) { }
 
-  CAN.setMailboxMask(4, Mask29Bit);
-  CAN.setMailboxMask(6, Mask11Bit);
+  CAN.setFilterMask_Extended(Mask29Bit);
+  CAN.setFilterMask_Standard(Mask11Bit);
 
   for (int c=16; c <= 23; c++) {
     CAN.setMailboxID(c, 0x0100);

libraries/Arduino_CAN/src/R7FA4M1_CAN.cpp

@@ -188,9 +188,16 @@ void R7FA4M1_CAN::end()
 }
 
 
-void R7FA4M1_CAN::setMailboxMask(size_t const mailbox_group, uint32_t const mask)
+void R7FA4M1_CAN::setFilterMask_Standard(uint32_t const mask)
 {
-	_can_mailbox_mask[mailbox_group] = mask;
+  _can_mailbox_mask[6] = mask;
+  _can_mailbox_mask[7] = mask;
+}
+
+void R7FA4M1_CAN::setFilterMask_Extended(uint32_t const mask)
+{
+  _can_mailbox_mask[4] = mask;
+  _can_mailbox_mask[5] = mask;
 }
 
 void R7FA4M1_CAN::setMailboxID(size_t const mailbox, uint32_t const id)

libraries/Arduino_CAN/src/R7FA4M1_CAN.h

@@ -54,7 +54,8 @@ class R7FA4M1_CAN final : public HardwareCAN
   bool begin(CanBitRate const can_bitrate) override;
   void end() override;
 
-  void setMailboxMask(size_t const mailbox_group,uint32_t const mask);
+  void setFilterMask_Standard(uint32_t const mask);
+  void setFilterMask_Extended(uint32_t const mask);
   void setMailboxID(size_t const mailbox, uint32_t const id);
 
   int enableInternalLoopback();