For decades, a tax return or payment postmarked by the deadline has been treated as filed on time — the “mailbox rule.” That rule hasn’t changed. What changed is the postmark itself: under new Postal Service rules, the date stamped on your envelope may be a day or more after the day you mailed it. If that date lands after the deadline, your return can be considered late.

What changed

Under the tax law, a return or payment that is postmarked by the due date generally counts as on time, even if the IRS receives it later. The postmark date is what matters — not the day you dropped the envelope off.

Under a Postal Service rule that took effect in late December 2025, a postmark shows the date mail is first run through automated processing at a regional facility — not necessarily the day the Postal Service picked it up. According to the National Taxpayer Advocate, that gap can be one to three days, especially for mail sent from rural areas or from towns far from a processing center.

So an envelope dropped in a blue mailbox on April 15 could be postmarked April 16 or 17 — and a postmark even one day late can bring late-filing or late-payment penalties.

Who this affects most

• Anyone mailing a check with an extension or tax return
• Paper-only forms that can’t be e-filed, such as Form W-7 and Form 706 (estate tax)
• Forms many people still mail, such as Form 709 (gift tax) and Form 3520 (foreign gifts and trusts), where late-filing penalties can be large
• Replies to IRS or state notices with a response deadline

How to protect your filing date

1. File and pay electronically whenever you can. You get an immediate confirmation, and postmarks don’t matter. Payments can be made through IRS Direct Pay or EFTPS.
2. If you must mail, go inside the Post Office. Ask the clerk for a manual (hand-stamped) postmark — it’s free and carries the date you hand it over. Postage bought and applied at the counter also shows the acceptance date.
3. Get proof of mailing. Certified Mail, Registered Mail, or a Certificate of Mailing gives you a receipt showing the date the Postal Service accepted your envelope. Keep it with your tax records.
4. Or use an IRS-approved private delivery service. Certain FedEx, UPS and DHL services qualify. Note that they can’t deliver to IRS P.O. boxes, so check the street address for your form.
5. Mail early. Don’t count on the last day.

What doesn’t count as proof

Postage printed at home, bought online or at a self-service kiosk, or applied with a postage meter shows only when the postage was printed — not when the Postal Service accepted your mail. Those labels alone won’t prove you mailed on time.

How we handle it for clients

We e-file returns whenever the IRS allows it, and we can help you set up electronic payments. For forms that still have to be mailed — such as Forms 3520, 706 and 709 — we’ll make sure you know the mailing deadline and how to get proof of mailing. See our pages on Foreign Trusts and Foreign Gifts and Trust and Estates, or if you’re behind on filings, Unfiled Tax Returns.

Current clients can message us through the client portal with questions. New to our firm? Take our 1-minute fit check.

This article is general information, not tax advice for your situation. Rules change; contact us before acting.