Sewer Backup. Who is responsible for damages?

My first problem as a park owner…

I have a park with a lagoon. There is a neighbor on the sewer also. The main line goes through his property and when it was built, he had the rights to use it in exchange for any payment for the easement or any other money.

So there was a back up through the plumbing into a bldg on his property due to a blockage in the main line. My insurance company’s first response is unless there it neglagence on my part they won’t cover anything under my general liability policy. If that is true, what insurance would I need to cover this in the future? Or should the neighbors insurance be covering this?

Who is responsible here for damages?

Other than this things are going very well.