Just use the return address on the mailing envelope.
Since Form 3531 is merely a boilerplate form on which an IRS employee checks off documents requested of a taxpayer, there's not typically an address on the document, as there is on most IRS letters and notices.
It's sent from the service center processing your return, so that's where the response needs to be sent.
If you no longer have the envelope, see the IRS' Where to File Paper Tax Returns With or Without a Payment page to find out the address appropriate to your state.
If you have any questions about the form, call 1-800-829-1040.
@edna4
