There's zero need for mail and mailboxes.

Just add the functionality to retainers. You summon your retainer at a Bell, add an item to a new section, let's call it 'Delivery'. You type in the name of the person to send it to, and off it goes. The person receives a notice, and when they check their retainer, they now have the items in the Delivery section of their retainer.

Exact same functionality as mailboxes, but expands on existing systems without adding unnecessary and superfluous extras.