When you first join the server, an item inbox is automatically created for your character. This allows other players to directly send you items. At the same time, you are able to send items to other players.
/post <player> Opens up a player's inbox, which you can drag and drop items onto empty spaces. If there is a piece of glass already in a slot, it means that there is an item already existing there awaiting collection. Once you close the box, a dialog is sent to you and the player receiving to let you know the transaction is complete.
/inbox opens up your own inbox, provided you have an item awaiting collection in it. If it is empty, you'll only receive a chat dialog to let you know this is the case.
/inbox upgrade allows you to expand your inbox by another 9 slots, up to a maximum of 54 slots (the size of a double chest). Each upgrade costs 25 experience levels, however this upgrade is permanent.
#@Cc_HTML~Warning:~@# Do not send rocket boots to another player's inbox, as this will permanently break the boots.