Courier

/letter [text]

 * Creates a Courier Letter with the supplied text. Hold the Letter in your hands and write /letter [text] again to add more text to the Letter. Use &nl or \n to insert newlines. Multiple page Letters are supported, use right-click and left-click to turn the page.

/post [playername]

 * Sends the Letter held in your hands to another player, name verified to be a player on the server. Depending on the config, the received Letter will be privacy-locked to that player (can be unlocked by simply adding text to it with /letter).

/courier fees

 * Lists the fees, if any, that apply for sending mail or creating letters

/courier unread

 * Requests re-delivery of all unread mail, even if they have been delivered to you before. Courier knows the difference between a mail having been delivered and actually read, and this is meant to be used in the cases where you've managed to misplace the Letter without having read it first.

/postman

 * Intended for OPs only, if postman delivery happens seldom and you want to force one to appear. NOT NEEDED in actual gameplay.

Website
http://dev.bukkit.org/bukkit-plugins/courier/