You can find the mailman sleeping in the Post office in Paris on it's little isle to the left of the Musée Du Louvre and between the two south exits of Paris.

You will have to discover the key to his office to be able to wake the mailman up. Of course you will also have to help out sorting (minigame!) and also delivering the mail.

The mailman also delivers the mail at the Dream Carnival and will be glad to meet you there too.