(Disclaimer ---> I'm not a lawyer, and this is just my opinion).
Pretty touchy. If the postal service is using the name (like Happyville, Texas), then their Trademark is becoming diluted. However, it is still valid. The trademark holder can try to defend their trademark, and the case would need to be examined based on the details (your intentions, overlaps with their business, etc). If I registered HappyvilleChat.com and used it to chat among my neighbors, I would probably be able to win any challenges -- that is, I would have a vested interest in the domain and would be using it in good faith. If I registered HappyvilleBuilders.com and I lived in another state, I would lose.
If you think there is very high value in your potential name(s), consult a lawyer.