This hexagonal box stands 6 inches high and measures six inches across at the vertices. Both the box and lid that covers it are made of marble with ornate silver filigree decorating five sides (with the sixth side - the "linking" side - left undecorated). At each vertex is a 250 gp emerald which can be covered with a small marble sliding cover.
The six emeralds serve as the controls to link containers and to determine target destination of items.
The two emeralds on the undecorated side allow two Mystical Mailboxes to be linked with each-other. With those emeralds closed on both boxes, set the two boxes side by side with the pairing sides touching. On both boxes, set a matching pattern of opened or closed emeralds on the other four vertices (the "destination" vertices). With matching patterns set, open the two pairing vertices. This will result in a brief green glow of all open vertices. When that glow fades, the boxes are paired.
After linking, any item inserted into either box when the destination vertices are set in the pattern selected at linking will arrive in the companion box after six minutes.
Since there are four destination vertices and each has only two states (open or closed), any Mystical Mailbox can be linked with up to 16 other Mystical Mailboxes.
Linking a new mailbox using an existing destination pattern results in the previous linking for that pattern being broken.