Dilute some normal bleach in hot water and with a cloth rub all round the seal, every bit you can touch.
Take out the drawer and take it to pieces. Wash in the bleach solution - I find a bottle brush to be excellent for this.
Use bottle brush, or cloth, to clean as far back inside the drawer cavity as you can - top, bottom and sides.
Get a suitable implement such as a cable tie and clean out the jets in the roof of the cavity.
Reassemble and replace drawer.
It is important that you use biological powder for the next step - not liquids or anything other than powder. The cheapest own brand is fine.
Run the washing machine empty on the hottest setting using the powder. This will kill any nasty bacteria lingering in the machine. Bacteria love washing machines as they are warm and damp and cause it to smell.
If you normally use non-biological detergents and/or low temperature settings you need to clean your machine in this way every couple of months.