EXT coughed up some electrons that declared:

Yes, since the door is water tight, it is important to leave the door open
between uses to enable the washer to dry out when not used. Our washer, an
LG, has a sterilizing cycle that it recommends being used every so often
without clothes nor detergent, I can only assume that it is intended to
clean out the mould and/or mildew from washer, or to help prevent it from
building up.

I'm a Brit and we use nothing but front loaders (people like them under
kitchen worktops and so forth, plus our houses are comparatively tiny so
accommodating a top loader is harder).

To the OP:

I agree with EXT's point above, amongst others. The correct way to treat a
front loader is:

1) Leave the door ajar most of the time, as previously mentioned - makes a
big difference;

2) As above, use a boil wash (95C/200-whatever F) once in a while (boil your
towels or something if you don't want to "waste" this cycle)

3) If all else fails, a bit of hypochlorite bleach doesn't do any harm once
in a while but don't over do it or it may weaken the rubber components.

I suppose the problem may be worsened if in a very humid climate, but
England's not known for being bone dry. If 1-3 above don't solve the
problem, a wipe round the rubber with a rag after each wash might help -
particularly try to remove the puddle of water that collects at the bottom
of the door seal. I recommend cleaning out the drain filter regularly too,
if your machine has one.

HTH

Cheers, Tim