post foreclosure sewer and solid waste charges

I lost my home due to foreclosure. I received a 6-month delay of sale in January 2013. In July 2013 I stopped the utilities and mailed the keys to the mortgage company. They sold the property six months later. I keep getting billed for solid waste and sewer charges for the six months between delivering the property and the sale. Is it my responsibility to pay this? I kept up utilities and insurance and maintenance until the day my delay of sale expired. This was in Iowa, if that makes any difference. Thanks!