Post office offering emails is an interesting idea if you extend it further in the physical world. As in, using this identifier to deliver correspondence/parcels as well.

pros:

- privacy. Senders have zero idea where you actually are. mapping to physical addresses is performed by the post.

- no need to update addresses in a million accounts when you move, your email points to the new physical address automatically (no idea how that works in other countries, but here you can set automatic forwarding for at most 1.5 years after you move).

cons:

- goods being sent to gmail addresses