You walk right up to that neighbor's door and tell them that someone's been stealing your mail for weeks now and dumping it on their lawn. You show them a piece of mail that you absolutely need or were waiting for and ask them "why on earth would I intentionally dump this on your lawn when I need it so badly?" Ask them why, if they were putting up with it for weeks, they didn't come to your door and tell you anything before.
Um, yeah, what glassycans said. Clearly your neighbor is challenged in the mental department so you need to spell things out nice and clear for them.....