Bacteria build up on the door seal
Food deposits can find their way into the tight folds of the seals on the doors of your appliance. Bacteria builds up on the deposits and the effect of this can be a really bad smell. Simply clean the seals with some anti bacterial spray and wipe over. In most cases the seal can be removed from the door by carefully pulling it out and given a more thorough clean. Push the seal back into the door fixing groove when its all clean.