collection agency/wrong address

I get many collection notices for someone who lived in the house I now own,she hasn't lived here in over 8 years.I try to return to sender but they keep coming.Sometimes I am so frustrated I open them and call the agency to politely tell them to stop sending to me.This happened again today and a very rude man on the other end told me that since I opened mail addressed to someone else,I have now assumed the responsibility for her (considerable) debt.I tried to explain that the return to sender seems to do no good,and was only trying to be helpful to this collection agency,but he insists that I now have to pay this debt.He is full of s**t,isn't he?