If you do not want to throw it out,I would not use it until it had been completely dried out. The bad smell is caused by the bacteria produced by the anaerobic condition of the mix over the long time with no oxygen. If it were me I would throw it on the garden, not use it for any propagation. Al

I've heard that baking mix in the oven (in a glass dish) for about a half an hour to forty five minutes at 250 degrees will kill fungus, but I'm not exactly sure what's causing it to smell bad.

The smell is caused by bacteria growing in it, just like if you had a bucket of kitchen waste full of water.

If it's worth it to try to keep it, spread it out fairly thinly on something that drains well. Vinyl window screening cut to fit the bottom of some flats comes to mind. And sit them out in the full sun until they're dry. If it's going to rain again, put them under shelter.

You CAN put them in the oven, but the smell is worse than you can believe -- really bad!

If it isn't crucial that you use this particular bag, just toss it into the garden like Calistoga suggested.

If you have seeds that are very important to you, don't use this soil to start them.

And keep your potting soil out of the rain.

Sue