I have had this problem for years, no one can seem to help --- I know its not the washing machine itself because we just installed a new one in 4/08 -- 1st of all its in the basement so it needs something called a check valve which we thought was the white plastic contraption that the hose from the washer feeds into before going into the main drain--I was told that this check valve is for making sure the water goes out but does not come back in (thus avoiding the discusting mildew smell that has plagued my laundry for years ) well, my husband took the check valve apart and its just an empty plastic thing? Is there supposed to be something inside it?

Never hear of a check valve on a washer before. Unless the connection between the washer discharge hose and the washer stand pipe is a sealed connection it wouldn't do any good any way. If you have water backing up the stand pipe you need to snake or otherwise clear the drain line. Unless you have water periodically overflowing the stand pipe it isn't the source of the mold or mildew. Yes, there should be some kind of flapper inside the check valve. Sounds like you were mislead a long time ago.