The IRS uses Notice CP503 to inform you for the 2nd time that they expect payment of past due taxes. Your situation is not yet urgent but needs correction by the tax attorney. There is also a Notice CP503K & a Notice CP503H, concerning Health Care Tax liability.
The next notice from the IRS will likely be the Notice CP504, Notice of Intent to Levy. So, it's best to take care of this problem prior to the IRS seizing your wages or property. Please follow the Prep Steps for IRS Notice CP503 and see Mr. Hopkins in Colorado Springs, Pueblo or Denver before proceeding!