Two things usually prevent the AC drain from draining: MOLD or DEBRIS.

If its debris, take the blower motor (behind the glove box) out and clean out the air box.

If its mold, take out the blower motor and (carefully) spray 1:1 bleach water in the airbox and on the little fins of the evaporator. Get under the car and get one of those really long handled mini-bottle brushes and go to town on the drain tube. You should get a flood of water when the tube unclogs.

For a MILDEW smell only when the a/c is on, run up to the store and pick up some Rug Doctor Odor Remover and take out your blower behind the AC. Yet again, mix it 1:1 with water in a spray bottle and liberally spray down the airbox, evaporator, AND what you can of the blower motor squirrel cage. One application usually does the trick. I found the Rug Doctor Odor Remover works great totally by accident (grabbed the wrong bottle from under the sink but the stuff works great!).