does the discharged drain water go into an stand pipe or or connect to a spigot underneath the sink?
if into a stand pipe ok.
if connected to spigot on trap then thats your problem !
whatever discharges down the sink drain will certainly bring back dirty water to machine or get smells back from waste drain.
you can buy a simple fitting called a drain away kit that comes in the shape of a plastic clamp , once fitted on waste pipe under sink it comes with it's own cutting tool to bore hole in waste pipe.
remove cutting tool & screw in adapter fitted with it's own non return valve to stop any smells or water getting back into machine. hope this helps.
gary
April 2009