The Mailman (Postman in the UK) is always seen wearing his blue mailman/postman uniform. He has a mustache and is usually (almost always) wearing his mailbag slung around one shoulder.
The Mailman/Postman first appeared in the episode Special Delivery when delivering Walden's package to Wubbzy. And he had an important role in Broken Record, which appears to be chased by a Buzzlebug. The last episode the Mailman/Postman appeared in was Run for Fun, where he ran in the Wacky Dash while still carrying his mailbag.