Our friends at the IRS continue to be way behind with processing their mail. Believe it or not, that includes many checks that the IRS has received in the mail since COVID hit last March.
For that reason, if you pay quarterly estimates, consider making those payments online at https://www.irs.gov/payments. I started paying my quarterlies online using direct debit last year during COVID and found the whole experience preferable to writing and mailing them a check along with a payment voucher 1040-ES.
The IRS has issued more info on What Taxpayers Need to Know About Making 2021 Estimated Tax Payments.
And remember, third quarter estimated tax payments are due 9/15/21.