A little-known change by the U.S. Postal Service at the end of 2025 has created a new compliance risk for taxpayers, small business owners, and tax professionals—especially those who still rely on mailing tax returns or payments.
As of December 24, 2025, the USPS implemented Rule 608.11, which changed how postmark dates are applied to First-Class mail. While the IRS’s filing rules themselves have not changed, the way postmarks are determined has—and that difference matters.
The IRS Mailbox Rule Still Applies
Under Internal Revenue Code §7502, often referred to as the “mailbox rule,” a tax return or payment is considered timely if it is postmarked on or before the filing deadline.
In any dispute with the IRS, the postmark date controls. Not the day you dropped the envelope off. Not the day you intended to mail it. The postmark.
Historically, this wasn’t an issue because the postmark usually matched the day the mail was accepted by the local post office. USPS Rule 608.11 changes that assumption.
What Changed Under USPS Rule 608.11
Under the new rule, the postmark may now reflect the date the mail is first processed at a regional sorting facility, rather than the date it was dropped off or accepted at a local post office.
Because mail is often transported to processing centers days later—especially during peak periods like tax season—the postmark date may be one or more days after the actual mailing date.
That means a return or payment mailed on time can still receive a late postmark.
Why This Creates a Tax Compliance Risk
The IRS continues to apply the mailbox rule strictly. If the postmark is dated after the deadline, the IRS may treat the filing or payment as late—even if the taxpayer mailed it before the due date.
This can result in:
- Late-filing penalties
- Late-payment penalties
- Accruing interest
- Delayed or denied refunds
- Missed statute-of-limitations deadlines
Simply stating that something was “mailed on time” is no longer sufficient without documented proof that satisfies IRS standards.
Mailing Methods That No Longer Provide Protection
Many taxpayers assume that any visible date on an envelope proves timely mailing. Under USPS Rule 608.11, that is no longer true.
The following methods do not reliably establish proof of timely mailing:
- Postage printed at home
- Self-service kiosk postage
- Meter strips or pre-printed labels
These methods do not confirm when the USPS officially accepted or processed the mail, which is what now drives the postmark date.
Best Practices to Reduce Risk
Given the increased uncertainty around mailed postmarks, taxpayers and practitioners should adjust their filing and payment practices.
Recommended best practices include:
- E-file tax returns and submit payments electronically whenever possible
- Mail documents several days before the deadline—not on it
- Use Certified Mail and retain the white, date-stamped receipt
- Request a hand-canceled postmark at the post office
- Use approved private delivery services (such as FedEx or UPS) that meet Treasury regulations
Electronic filing provides immediate confirmation that the IRS has received the return or payment and eliminates mail-processing delays entirely.
Why This Matters for Business Owners
Small business owners, S-corporation shareholders, and taxpayers who make estimated payments are particularly exposed under this rule change.
Late-filed returns or payments can trigger penalties even when cash flow, payroll, or entity-level compliance is otherwise handled correctly. The issue is not tax strategy—it’s administrative timing.
As enforcement becomes more automated, reliance on informal IRS leniency is not a sound approach.
2026 Filing Season Reminder
The IRS will begin accepting e-filed individual tax returns on January 26, 2026.
With USPS Rule 608.11 now in effect, early preparation and electronic filing are more important than ever. Waiting until the deadline and relying on the mail introduces avoidable risk.
Bottom Line
The IRS mailbox rule has not changed—but the way postmarks are applied has.
Under USPS Rule 608.11, mailing tax documents on or near the filing deadline is no longer a safe fallback. Filing electronically or mailing early with documented proof is the best way to avoid penalties, interest, and unnecessary disputes.
Tax compliance depends not only on what you file, but also how and when you file. Understanding administrative changes like this one is key to staying compliant in today’s filing environment.
