When you leave Google, your existing mail account becomes a black hole - all mail sent to the address silently goes away. There's no bouncing or auto-responder. No big deal, right? Anyone at Google can probably get your new address off go/epitaph.
BUT: If, like me, you worked with anyone outside of Google (for example, patent attorneys), they might not know about your departure, and they can haplessly send email into the black hole for months without discovering that you're not there any more. I found this out recently when a lawyer who had been trying to reach me all summer finally found out that I had left the company.
Simple fix: before you leave, you can file a ticket to have an auto-responder set up. If you've already left, your HR Business Partner can help you out.