It depends on what you are mailing to them. If you're mailing a tax return, go to Where to file paper tax returns with or without a payment.
For filing a federal tax return?
Go to this IRS website for address on where to mail the federal tax return, by state, with and without a payment - https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment