Every time my washer drains it smells like rotten eggs or dirty diapers. It only does it when draining. What can we do to treat this problem and prevent it from happening again?

Try putting some chlorine in the drain

By "chlorine" Bob means bleach.
Try this; tonight pour a half gallon of bleach down the washer standpipe and let it set over night. Next morning flush the line out with a few large pans of boiling water,(this is important) The b leach will start to breakdown any build up and make it slippery and the boiling water will loosen the grease and wash it away.
Smell better now? Good luck, Tom

When I washed my clothes, I had the same problem... If you take apart the fabric softner holder all the fabric softer that didn't drain clogs up the holder and receiver and becomes like grease in there. It's the most disgusting smell I have ever come across and It almost made me vomit when I was cleaning it. It smelled like java the hut went #3 in their, that's a combo of #2 and #1. haha.. it will be like grease... very hard to remove you'll need to have a high tolerance to smells to do it and a rinse won't help, you'll need a degreaser or something I tried to clean it with bleach and soap and it did nothing. I used a sock on my hand and manually cleaned it and threw away the sock... Good Luck.