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.

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.

Please select this link to read the complete article from OSAP Mission Partner Clark Schaefer Hackett (CSH).