I have seen the topic of mystery terrible smells from peoples kitchen sink and washing machine on various sites. I have suffered with this as well and may have discovered one reason. i recently ordered a new washing machine, in preparation for delivery i unplugged it, disconnected the hot and cold water pipes but also pulled the drainage pipe out from the under sink U bend connection. The washing machine drainage pipe stank foul, i also unscrewed the plastic cap that screws onto the U bend and has on it's other side a tapered pipe that the washing machine drainage pipe slots onto, it was full of the most disgusting combination of grease and sludge i've ever encountered, ( my heart goes out to sewer workers ) this stuff had hardened over time, stank disgusting and was awful. It occured to me that some of the stuff that goes down the sink outlet U bend might leak into the washing machine drainage pipe, and looking on a plumbing site see this is possible. I've decided to drain the machine directly into the sink (drainage pipe into sink ) true you have to be careful to remember to place the pipe in the sink each time you use the washer or flood the kitchen, it's not much effort for the hygiene gain and to avoid that terrible smell. Regards all Adrian.