For all individuals who filed an extension of time to file their personal income tax return, today is the last possible day to file that return without incurring a late filing penalty. While over 90 percent of Americans e-file their tax returns, those who paper file often simply put their tax return in an envelope with stamp on it and mail it to the IRS. This is a mistake that could prove very costly.
Placing a stamp on an envelope isn’t enough to prove the date it was mailed. (AP Photo/Paul Sakuma, File)
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Fifteen days into the US government shutdown, one imagines tumbleweeds blowing through the IRS offices where tax returns are normally processed. When US workers return to work, they will be kept very busy scanning and processing these paper filed returns. The date a tax return is mailed is important: taxpayers who file late will face a penalty of 5% of the unpaid taxes for each month or part of a month that the return is late, up to a maximum of 25% of the unpaid tax. Coupled with interest, which accrues on both tax and penalties, this can add up quickly.
Just pooping the return in the mail, however, isn’t enough. It won’t prove that a tax return was mailed today, October 15, and timely filed. In August of 2025, the Post Office proposed a rule that would alert US citizens to a little-known fact:
“[W]hile the presence of a postmark on a mailpiece confirms that the Postal Service was in possession of the mailpiece on the date of the postmark’s inscription, the postmark date does not inherently or necessarily align with the date on which the Postal Service first accepted possession of a mailpiece.”
In other words, the postmark date does not reflect the date that the Post Office actually received a piece of mail. I became interested in the issue of timely mail delivery and post mark dates after a good friend of mine recently sent out wedding invitations. She dropped them in the mail in early May of 2025, but got almost no RSVP’s by July. No one had received the invitations. Suddenly, in late August, the responses started coming in. The date a wedding invitation is mailed is important to the happy couple, but doesn’t carry the same consequences as a tax return. Some reports show that nearly half of the mail is postmarked as least a day after it is received by the post office.
The consequences are far more dire for those who file a refund claim. Claims for refund must be filed by the taxpayer within two years of the date a return was filed, or three years from the date the return was filed – no exceptions. If the post office doesn’t put a postmark on the piece of mail the date it is received, a valid claim for refund could be completely disallowed and rejected as untimely if it was filed on the last day.
My practice is to make sure for all IRS correspondence that has a deadline, go and wait in line at the Post Office and send the mail certified, return receipt requested. I get the stamp on the receipt and I will later get a green card that establishes both the date of mailing and the date of receipt by the IRS. This is best practice and everyone who needs to be able to prove the date something was mailed should follow it.