Help! mail problem

I do work for someone who wanted a mass mailing to go out to a huge list of contacts gathered over a couple of years. As might be expected there are a lot of dead addresses. The problem is even though I am running the php program from their server (via a webpage), the returns are returning to MY default e-mail address. I used the from:theiraddress@theirsite in the mail statement and still I get the returns. How do I get my address out of the picture. I don't want my address clogged w/ their stuff. Ed