Our new sink drain (not the disposal side) is over run with mildew in the seal and stainless drain insert? We did not leave any standing water or dishes in it. I have treated it with Clorox/stainless cleaners and nothing is working? Do you have any suggestions?

What seal do you refer? There is no viseable seal on a drain, may a pull out plug. Try mixing 1/4 cup table salt with 1/4 cup baking soda and dump in the drain followed with a cup of vinegar. Let it soak all night and tomorrow dump a bucket of really hot water with a cup of fresh bleach. Let me know in a month if its still clean.

When I refer to the seal--I mean-the strainer basket side. The mildew has gotten in to the silicone or putty used to attach this to the drain and sink. It looks more like putty than silicone actually. Also there are signs of mildew on the removeable drain basket as well. Does that make more sense. I will try your advice with the salt/baking soda and vinegar tonight. Thanks! Maybe my strainer basket needs to be removed and re-sealed?