These units have a gasket in the front, right inside the door, that holds a little bit of water after a cycle. Stick your finger in and pull the gasket back a bit, you'll probably see yucky black mildew everywhere.
You can leave the door open all the time and this will help cut down on mildew building up in there, but I have found that leaving the door open isn't enough. I have a dehumidifier next to the washer that runs on auto, helping to dry out the washer. Plus, once every six months or so I have to run a rinse only cycle, and when it is in the middle of the cycle, press the stop button ONCE.
Then you open the detergent drawer and pour in about 4 gallons of hot water (I have a utility sink in the washroom that makes this easy). Follow that up with about 1 and 1/2 cups of bleach and then let it sit for 4 hours.
After 4 hours come back and press and hold START. The washer will complete the cycle and will have washed away most of the mold and mildew in the washer.
Its a crappy problem but common to front loaders.
Hope this helps, it keeps my clothes from smelling like the swamp.