The form is open on your screen. It's Form SS-4, the US tax agency's application for a company tax number. It's full of tiny numbered boxes, and two different lines ask for an address. You have a Florida company, a home in Buenos Aires and maybe a mailing address in Miami. So what address do you put on Form SS-4, and on which line?
This guide answers that one question for founders applying for an EIN, the US company tax number, from abroad. We'll explain each line in plain words, then how to send the form, because the online route is closed to most people outside the US. It's general information, not tax advice. For anything about your taxes, talk to a US tax professional.
First, the words
The IRS is the Internal Revenue Service, the US federal tax agency.
An EIN is an Employer Identification Number. It's a nine-digit tax ID for your company, a bit like a company tax number in your home country. Banks, payment platforms and clients will ask for it, even if you never hire anyone.
Form SS-4 is the IRS form you fill in to get an EIN (Form SS-4).
A P.O. box is a locked mailbox you rent at a US Post Office. A CMRA address is a private mailbox address, the kind a virtual office gives you, written with a "#" or "PMB" and a mailbox number, like "2125 Biscayne Blvd, Suite 300 #1234."
What address to put on Form SS-4: the short answer
- Line 4a and 4b: your mailing address. This is where the IRS sends letters. It can be a street address or a P.O. box, and it can be outside the US.
- Line 5a and 5b: a street address, only if it's different from line 4a. No P.O. box.
- Line 7a and 7b: the name and tax number of the responsible party, a real person. Without a US tax number, line 7b gets the word "foreign" or "N/A."
Now the detail.
Lines 4a and 4b: the mailing address
Line 4a is the street and number (or the P.O. box). Line 4b is the city, state and postal code. The form says this line can be a street address or a P.O. box. A foreign address is allowed too, with the country written out in full, like "Argentina," not "AR."
This is the most important address on the form. Why? Because the IRS mails your EIN confirmation letter here. It's called the CP575. It's one page, and banks often ask to see it. If it gets lost, the IRS won't print the original again. It sends a different letter instead, called Letter 147C (IRS, understanding your CP575). Since summer 2026, people who can use the IRS Business Tax Account online can also download a digital CP575, which the IRS says banks accept (IRS newsroom).
Later letters go here too. Notices about your filings, questions, penalties. So pick an address where you'll actually see mail quickly.
You have two common choices:
- Your home address abroad. It's allowed and it's free. But international mail can take weeks, and nobody tells you a letter has arrived.
- A US mailing address. A virtual office address can go here. We haven't found any IRS guidance on mailbox addresses in particular. But the form accepts a P.O. box on this line, which suggests a mailbox address works as a mailing address too.
Write a mailbox address exactly as your provider gives it. Say your number is 1234. Line 4a gets "2125 Biscayne Blvd, Suite 300 #1234" and line 4b gets "Miami, FL 33137."
Lines 5a and 5b: the street address, only if different
Line 5a says "Don't enter a P.O. box." It's for a street address, and only if it's different from what you wrote on line 4a. If your mailing address on line 4a is already where the business is run from, leave lines 5a and 5b empty.
Can a mailbox address go on line 5a? The IRS doesn't say. Nothing official we've found answers it. Our honest view is that this line is about where the business physically is, so we wouldn't put a mailbox address here. A Florida company can be run from anywhere, and the place you actually run it from is the natural answer. Our guide on the three addresses Florida asks for explains the same idea on the state's forms.
Lines 7a and 7b: the responsible party
The responsible party is the person who really controls the company. The IRS says it must be an individual, a real human, not another company (SS-4 instructions).
Line 7a is that person's name. Line 7b asks for their US tax number: an SSN (Social Security number) or an ITIN (Individual Taxpayer Identification Number, a tax number the IRS gives people who can't get an SSN). Most founders abroad have neither, and that's fine. If you don't have one and aren't eligible for one, you write "foreign" or "N/A" on line 7b (IRS, responsible parties).
Whether you need an ITIN for other reasons is a question for a tax professional. The EIN application itself doesn't require one.
If the responsible party changes later, the IRS wants to hear about it within 60 days, on Form 8822-B.
How to send the form from outside the US
You can't use the online EIN application if you have no home, main place of business or main office in the US (IRS, Employer ID numbers). The online tool also asks for the responsible party's SSN or ITIN. And a mailing address in Miami doesn't change where your business is really run from, so it doesn't open that door.
Here's what's left, as of October 2026:
- Phone. Call 267-941-1099, Monday to Friday, 6 a.m. to 11 p.m. Eastern time. It isn't a free number. The caller has to be authorized to receive the EIN and able to answer the questions on the form, so have it filled in before you dial.
- Fax. Send it to 304-707-9471 from outside the US. Include a fax number for the reply and the IRS says you'll have the EIN in about four business days.
- Mail. Send it to EIN International Operation, Cincinnati, OH 45999. This takes about four weeks.
Whichever way you choose, the paper CP575 still goes to the mailing address on line 4a. That's one more reason to choose that line carefully.
If you need to change the address later
Moved? Switched to a US mailing address? You tell the IRS with Form 8822-B, a short paper form for a new mailing address, a new business location or a new responsible party (about Form 8822-B). It can't be filed online. The IRS says it takes 4 to 6 weeks to process. We go through it step by step in how to receive IRS mail while living abroad.
And remember that the state of Florida keeps its own record. Changing your address with the IRS doesn't change it on the state's site, and the other way round.
Where Prima fits
Prima gives you a US mailing address for line 4a: 2125 Biscayne Blvd, Suite 300 # plus your mailbox number. Our team receives your mail, photographs every envelope for the app, and scans legal and government mail at no charge. So the CP575 doesn't sit in a pile for a month. Before your first letter arrives, you'll sign USPS Form 1583 online, which our Form 1583 guide explains.
Prima doesn't apply for EINs or fill in tax forms for you. We're the address and the mail. Plans start at $39 a month, and you can see them here. If the EIN is one step on a longer list, our checklist of what to do after forming a Florida LLC shows what comes before and after.