If a taxpayer fails to respond to the CP 501 notice, that taxpayer may receive a CP 503 IRS notice, which indicates that the amount listed in the CP 501 remains outstanding, and the IRS has not heard from the taxpayer regarding that amount. This is typically the last general warning a taxpayer will receive before the IRS initiates a process whereby the IRS will seize certain assets in order to satisfy the tax obligation.

Failure to respond to a CP 503 IRS notice may lead to the taxpayer’s receipt of a CP 504 IRS Notice, which indicates that the unpaid amount remains outstanding, and failure to pay may result in the seizure of the taxpayer’s state income tax refund, which would be applied to the outstanding tax obligation. Internal Revenue Service, Understanding Your IRS Notice or Letter. Alternatively, the taxpayer may receive a CP 504B, which again states that an outstanding tax obligation has not been met, and that the IRS intends to seize (levy) certain property or property rights in order to apply the proceeds to the outstanding tax liability.