The door seal rubber isn't the cause of the smell.
If you run a rag under the seal, where it meets the inner drum, you'll see it's coated in gunge.
This is lining the inside of the outer tub.
It's going rancid, causing the smell and the mould on the seal is part of the result.
You need to carry out several maintenance washes to clean up the machine.
You'll also need to obtain and use a washing machine cleaner, you can get this in the likes of Asda and Tesco.
First, with no laundry, put the machine on it's hottest wash and run that program.
Do this a few times.
It will dissolve the build up of soap, conditioner and body fats.
Then, use the washing machine cleaner.
Once you get it cleaned up, wiping the door seal with a bleach solution will slowly improve it's looks. Alternatively, you could buy and fit a new door seal.
If you then dry the door seal after use and leave the door open, as well as doing a regular maintenance wash, it should keep the machine clean and fresh.
Check it out, see how it goes and let me know if you need more help.
Hope this helps