Mailboxes is a simple mod allowing you to send letters with attachments to your friends.
Craft a letter and right click to edit.
Save changes by clicking on cross.
Escape will reset the entire letter.
Attach items with /attachitem <Letter title> <slot (0 or 1)>
Validate with Finish Button. Warning: You can't attach items to a validated Letter !
Craft a Letter Box. To send the letter, run /sendletter <Target Player> or /sendletter name <target name> if the player is not connected.