Mailboxes are a convenient, quick way to send mail between other players on a server. While the exact delivery mechanism remains unclear (it's possible that deliveries are made by specially trained Postal Bees), letters inserted into a mailbox are rapidly transferred to the recipient's mailbox for pickup.
Any player wishing to join the mailing network need only place a Mailbox down in the world. Usually, somewhere easily accessible and near their home is preferred. Letters may be simply notes getting sent between players, although it is a trivial matter to include several items in a mail package. These items will remain safely ensconced within the recipient's mailbox until such a time when the owner of the mailbox removes the items and opens the package. Mailboxes can be set to private, which will prevent other players from seeing what is inside a mailbox in addition to preventing them from breaking them.