What this notice is about If you received a CP521, CP521 (SP) or CP621 Notice, the IRS is reminding you that you have an installment payment due. Send your payment immediately. View this notice and manage your communication preference online Sign in to your Online Account to: View and download this notice Go paperless for certain notices Get email notifications for new notices What you need to do Read your notice carefully. It's a reminder to send in your monthly payment. Pay by the due date. Go to the payments page to find out more about your payment options. Frequently asked questions What is the notice telling me? Your monthly installment agreement payment is due to us by a specific date. What do I have to do? Pay the amount due shown on the notice. Mail us your payment in the envelope we sent you. Include the bottom part of the notice to make sure we correctly credit your account. What happens if I don't pay? If you don't send us a payment, the interest and penalties you'll have to pay will increase and you could default on your installment agreement. Who should I contact? If you have any questions, call us at the number at the top of the notice. A customer service representative will assist you. Reference Tools Publication 1, Your Rights as a Taxpayer Full list of tax forms and instructions Need Help? You can authorize someone to contact the IRS on your behalf. See if you qualify for help from a Low Income Taxpayer Clinic. If you can’t find what you need online, call the IRS number at the top of your notice or letter. If you didn’t receive a letter or notice, use telephone assistance. If you can't resolve the penalty on your own, contact Taxpayer Advocate Service, an independent organization within IRS.