The once-relied-upon “mailbox rule,” the belief that when something important was mailed and considered timely once the post office received it, is gone.
A quietly released update (FR Doc. 2025-20740) from the United States Postal Service (USPS) is changing how postmark dates are defined and applied, and it could create real problems for taxpayers who rely on mailed submissions to meet deadlines. This matters because the IRS and many state agencies still treat the postmark date as proof of on-time filings for returns and payments.
What Changed?
As of December 24, 2025, the USPS added a new section to its Domestic Mail Manual that formally defines what a postmark is and when it applies. Under the updated rule:
- The official postmark date reflects the first automated processing operation at a USPS facility, not the date the mail was dropped off at a post office or mailbox.
- In practice, this means a tax return or payment dropped in a mailbox on the deadline could receive a postmark dated one or more days later.
- Manual postmarks or Certificates of Mailing may still be requested to support proof of timely mailing.
This update does not technically change USPS operations, but it clarifies how postmarks work and highlights processing realities that have grown more pronounced with regional sorting and transportation changes.
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Why This Matters for You
Many CPAs and tax professionals know that a tax return is generally considered timely if it is:
- Properly addressed,
- Adequately postage paid,
- Postmarked on or before the filing deadline, and
- Deposited with USPS by the due date or extension due date.
Under IRC §7502, the IRS uses the postmark date as evidence of the filing date when a return isn’t received by the deadline. However, the updated USPS definition now disconnects the postmark date from the physical act of dropping a document into the mail.
This creates a situation where a document mailed by a client on April 15 could bear a postmark dated April 16 or later if processing is delayed, even if USPS initially accepted the envelope on the deadline.
Practical Impacts for Tax Filing and Payments
Returns and Extensions: If a client mails a return with an April 15 deadline and it isn’t postmarked until the next day due to processing timing, it could be treated as late by the IRS or other jurisdictions. This exposes both clients and practitioners to avoidable disputes.
Payments and Penalties: Property tax, estimated tax, and other payments mailed close to deadlines risk being considered late if the postmark date lags. Some local treasurers are already warning residents to mail payments early or use online methods.
Charitable Contributions: For year-end donations, a late postmark could affect whether a gift counts for the intended tax year.
Act Now!
This update shifts more risk to taxpayers and preparers, making it essential for firms to revisit how they handle mailed filings, deadlines, and proof of timely submission. Here are some tips.
1. Advise Clients to File Early or Electronically
Electronic filing avoids postmark ambiguity. For payments that can’t be submitted online, consider certified or registered mail for more substantial proof of mailing.
2. Use Manual Postmarking or Certificates of Mailing
Tell clients to request a manual postmark at the retail counter or use a Certificate of Mailing when a deadline is near.
3. Update Internal Procedures
Train staff to understand and communicate the new postmark definition, especially for clients who still mail returns or documents.
4. Communicate Deadlines Clearly
Set internal deadlines well ahead of client deadlines if mailing is involved. A good rule of thumb is to treat USPS processing time as part of the deadline, not just the date it was dropped in the mail.
5. Review Engagement Letters
Consider including language that outlines mailing risks and options for proof of timely submission.
- Download the Postmark Change Marketing Kit to assist with client communication, social media posts, and more.
Reshaping Proof
This USPS update doesn’t change the law on when returns are due, but it does reshape how proof of timely mailing may be interpreted. For CPAs and tax professionals, early awareness and proactive communication are essential to avoid last-minute surprises for clients and unnecessary penalties.
If your firm still handles paper tax returns or payments, now is the time to revisit procedures and client guidance so deadlines remain clear and defensible.
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