Water backing up into washer when not in use

If water is getting into washer when not in use, then I would assume that the washer outlet pipe is connected to your sink waste outlet, this being the case, then this is the cause of your problem
What is happening is the washer outlet hose is below the bottom of your sink outlet pipe, and each time you empty the sink, some dirty water is syphoning back into the washer.thus causing the smell.
The solution is to raise the washer outlet hose above the bottom of the sink and waste pipe outlet so that the water cannot drain back into the washer.
It would be a good idea to remove the pipe from the washer where it fastens to the sink waste, and clean the connection of muck etc'
Plz rate my solution.
thanks.