The IRS is automatically waiving the estimated tax penalty for the more than 400,000 eligible taxpayers who already filed their 2018 federal income tax returns but did not claim the waiver. The IRS will apply this waiver to tax accounts of all eligible taxpayers. Earlier this year, the IRS lowered the usual 90% penalty threshold to 80% to help taxpayers whose withholding and estimated tax payments fell short of their total 2018 tax liability. The agency also removed the requirement that estimated tax payments be made in four equal installments, as long as they were all made by Jan. 15, 2019. Over the next few months, the IRS will mail copies of notices CP 21 granting this relief to affected taxpayers. Any eligible taxpayer who already paid the penalty will also receive a refund check about three weeks after their CP21 notice regardless if they requested penalty relief. The agency emphasized that eligible taxpayers who have already filed a 2018 return do not need to request penalty relief, contact the IRS or take any other action to receive this relief.