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Home > Guides > Mailing a W-9 to a client
Mail it or send it through the client's secure portal; do not send it by plain email. A W-9 carries your Social Security number or employer ID number, and plain email pushes that data through mail servers and inboxes you cannot control. A sealed envelope addressed to the right department works fine. Never send a W-9 to the IRS: the form belongs to the payer who will report your payments.
One honest caveat: LetterAgent cannot enclose your signed original form, since letters go out with no inserts, so mailing a signed W-9 is a do-it-yourself envelope. This guide covers how to handle it.
This is general information, not tax advice.
| Method | Security | Speed |
|---|---|---|
| Sealed envelope by mail | Strong; physical chain of custody | A few days |
| Client's secure portal | Strongest of the four; no email involved | Instant |
| Encrypted email or password-protected PDF | Better than plain email, still email-shaped risk | Instant |
| Plain email | Weak; readable on servers and in inboxes | Instant, and the riskiest |
This confuses almost everyone the first time. The W-9 is a request for your taxpayer identification number, and it travels from you to the payer, the client who will pay you. The payer uses it to file information returns, like the 1099-NEC, reporting what they paid you during the year. The IRS then matches the name and taxpayer number you wrote against its own records, a process called TIN matching. If the numbers don't match, the payer can be required to start backup withholding on your future payments. The IRS already has your taxpayer number from your own filings; it has no use for your W-9, so mailing one to the IRS just creates a stray copy of your Social Security number floating around a government mailroom.
Yes, with two conditions. Address the sealed envelope to the specific person or department who asked for it, "Attn: Accounts Payable" rather than a generic company address that lands in a shared mailroom pile. And keep a copy of the signed form for your own records, so you know exactly what taxpayer number is out there under your name. The postal system is a closed chain of custody compared to email: fewer copies, fewer servers, fewer strangers. If you prefer not to use the mail at all, a fax to a confirmed number is the old-school equivalent and still common in accounting departments.
Email usually travels and rests unencrypted on servers between you and the recipient, and inboxes get compromised routinely. A W-9 sitting in someone's sent folder is a Social Security number waiting for the next phishing email or data breach. If email is genuinely your only option, use an encrypted email service or send a password-protected PDF with the password delivered separately, by phone or text. Even that is a compromise: it protects the file in transit but leaves a copy of your number in two inboxes. A client portal upload avoids email entirely and is the best digital option when the client offers one.
The client can apply backup withholding to your payments, holding back a flat share for taxes until you provide a correct W-9. That is money you eventually reconcile on your own return, but it is cash flow you lose right now, and it flags you as a paperwork problem with the client's accounting team. Some clients go further and hold your payment entirely until the form arrives. A sealed envelope takes a few days; the withholding headache lasts until the next tax season. Send the form.
Ready to mail it? Send your letter with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and it is printed and mailed.
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