The Postmaster is a mysterious person or entity in World of Warcraft, that sends mail to characters to retrieve items that were not rewarded into the player's inventory, usualy because their inventory was full.
Besides the lost item, the mail also contains text:
We recovered a lost item in the Twisting Nether and noted that it was yours.
Please find the object enclosed.
If players are on the quest  , and their inventory is full when they succeed in winning from a Valiant-NPC, an in-game mail will instantly be sent to that character. The mail contains the lost item to be looted from the mail whenever the player wishes to do so.
The Postmaster seems to be some entity living in the Twisting Nether, collecting items that he finds are lost, and then returns those items to their rightful possessor. Due to the fact that he can judge wether or not items are lost, it is fairly safe to say that the Postmaster is a wise and perhaps even mighty entity. It's also safe to assume that his/her/it's alignment is neutral good.