If you are using Full CAN you will have option of Linking mailboxes in CAN configuration-->Receive Buffer Tab-->Last Column. (See the attached image). Do not check the Linking box in front of last mail box. For eg. if you want to link mailbox # 0 to 4, check the linking box in front of mailbox #0 to 3, but don't check that for mailbox #4.
Also all the linked mailboxes should have same AMR and ACR configuration. In short you need to have same ID configured for all the linked mailboxes.
When you link the mailboxes, you can access any mailbox and the respective mailbox will be empty. Once new message arrives it will go to empty mailbox . If two mailboxes are empty then the new arrived message will be stored in the lowest numbered mailbox.For eg. if mailbox0 and mailbox3 are empty, the new message will be stored in mailbox0.
Please feel free to update if you need any clarification.
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Thank you for the clarification and support.