If you mail tax returns or payments, you need to change how you handle deadlines.

A postmark now reflects when USPS processes the mail, not when you drop it in a mailbox.

In late 2025, the U.S. Postal Service clarified how postmarks are applied. A postmark now reflects when USPS processes the mail, not when you drop it in a mailbox. Because mail is no longer always processed the same day, postmarks can be applied one or two days later.

The IRS uses the postmark date as the legal filing and payment date. That means a return or payment mailed on the deadline can still be treated as late and trigger penalties, even though you mailed it on time.

This is not a tax law change — it’s a processing change — but the risk is real.

One clear outcome:

Do not rely on last-minute mailing to meet tax deadlines. The safest approach is electronic filing and electronic payments. If mailing is unavoidable, send documents early and use certified or registered mail so you have proof of timely mailing.

If you mail tax returns or payments, you need to change how you handle deadlines.