Wipe the seal with a bleach solution.
Then run the machine on the hottest possible cycle with no washing in it and use double amount of biological washing powder - it must be biological. The cheapest will do.
That should cure it. Washing machine manufacturers now say we should run this 'maintenance wash' at least once a month. This is because we tend to wash at much lower temperatures these days and the washing detergent (powder or liquid) is mostly bleach free.
The machine is getting as clean as it used to causing mildew and odours.