USPS Postmark Changes Create New Risks for Timely Tax Filings
New USPS Postmark Rules
For decades, taxpayers have relied on a simple rule when mailing tax returns or payments: if the envelope is postmarked by the filing deadline, the submission is considered timely, even if the IRS or state tax authority receives it days later. This principle, commonly known as the “mailbox rule,” has long provided reassurance for time-sensitive filings.
However, a recent change announced by the U.S. Postal Service (USPS) may undermine that assumption and create new compliance risks for taxpayers who continue to rely on traditional mail. In November 2025, the USPS issued a final rule clarifying how postmarks are applied to mailed items, formally defining what a postmark represents and when it is applied.
What Has Changed?
USPS has clarified that machine-applied postmarks reflect the date of the first automated processing operation, not necessarily the date the mail piece was dropped off at a post office or placed in a mailbox.
In practical terms, this means a return or payment mailed on the filing deadline could receive a postmark dated one or more days later if the item is not processed immediately. Processing delays are increasingly common as mail is routed through regional processing centers rather than local post offices.
Why This Matters for Tax Filings
The IRS and many state and local taxing authorities rely on the postmark date to determine whether a return or payment was filed on time. If the postmark date falls after the deadline, the filing may be treated as late, even if the taxpayer deposited the item on or before the due date.
Late filings and payments can trigger:
Penalties
Interest charges
Compliance notices and correspondence
This change does not eliminate the mailbox rule, but it makes standard USPS mailing far less reliable unless additional precautions are taken.
Best Practices Going Forward
To reduce the risk of late filings or payments, taxpayers should consider the following best practices:
File and Pay Electronically Whenever Possible
Electronic filing and online payments provide immediate confirmation and eliminate postmark uncertainty altogether. Most tax agencies strongly encourage electronic submission.
If Mailing Is Necessary, Take Extra Precautions
Use Certified Mail, which provides proof of mailing and delivery.
Request a manual (hand-stamped) postmark at the USPS counter.
Pay for postage at the counter to obtain a dated receipt.
Mail time-sensitive items earlier than in prior years to allow for processing delays.
Planning Ahead Is Key
As filing deadlines approach, taxpayers should revisit long-standing mailing habits and adjust accordingly. What worked reliably in the past may no longer provide the same level of protection.
Working with your CSH tax advisor can help ensure that returns and payments are submitted using the most secure and efficient methods available, and that unnecessary penalties and notices are avoided.
