The Ender Letter can be used to send other people on a server items and messages.
When you right click with an ender letter in your hand i will open an GUI where you can add items to the letter and specify the receiver. To send / receive letters you need an Ender Mailbox. To send a letter you simply shift right click on any ender mailbox. Letters you receive will appear in every Ender Mailbox you placed yourself.
Note that you can only remove items from a received Ender Letter, once the letter is empty it will vanish.