Effective December 24, 2025, the United States Postal Service (USPS) adopted final rules that revise how postmarks are defined and treated. These changes have important implications for taxpayers who plan to mail their tax returns and rely on postmarks to establish timely filing.
Under the new framework, the USPS has clarified and, in some cases, narrowed what qualifies as an official postmark, where and when postmarks may be applied, how postmark dates correspond to the USPS’s acceptance of a mailpiece, and which mailing methods provide valid evidence of mailing dates. Taxpayers who do not adhere to the updated requirements may face an increased risk that their returns will be treated as late‑filed.
The USPS finalized the newly added Section 608.11, “Postmarks and Postal Possession” (the Final Rule), as part of the Domestic Mail Manual. On August 12, 2025, the USPS published the proposed rule, 90 F.R. 38716, and invited public comment. The USPS noted that this process was not a traditional rulemaking process because the Final Rule does not make any changes to USPS procedures for applying postmarks to mail and instead provides key definitions. The Final Rule’s purpose is to document existing practices and provide clearer guidance on how postmarks work and how they may differ from a mailpiece’s actual date of mailing.
What is the Final Rule?
Under the Final Rule, a “postmark” is defined as a marking applied by the USPS to confirm its acceptance of custody of a mailpiece. The postmark displays the location of the processing facility or retail unit that applied it, which may not be the location where the USPS first obtained possession of the mailpiece. As a result, the postmark date is not necessarily the date the USPS first obtained possession of the mailpiece. The postmark date generally reflects either the date of the first automated processing operation that is performed on the mailpiece or the date that the mailpiece was first accepted at a retail counter. Because most postmarks are applied at processing facilities, the postmark date and location often differ from the date and place of initial acceptance of the mailpiece
On January 2, 2026, the USPS issued a public statement titled Postmarking Myths and Facts, reiterating that it has not changed its postmarking practices. However, the USPS explained that recent adjustments to its transportation network mean some mail may not reach the processing facility on the same day it is collected or dropped off. Consequently, the postmark date applied at a processing center may differ from the actual date the customer mailed the item.
The Final Rule aims to equip customers with enough information to adjust their mailing practices (e.g., by mailing earlier, requesting a manual postmark at a retail location, or purchasing a Certificate of Mailing).
Importance for tax filings
Internal Revenue Code Section 7502(a) provides that the postmark date is treated as the date a tax return or payment is delivered to the Internal Revenue Service (IRS), sometimes known as the “mailbox rule.” Mailbox rule regulations state that “the sender who relies upon the applicability of section 7502 assumes the risk that the postmark will bear a date on or before the last date, or the last day of the period, prescribed for filing the document or making the payment.”[1] Accordingly, the IRS maintains that taxpayers who mail their returns accept the risk that the postmark may reflect a date that renders the filing untimely. Neither the mailbox rule nor its corresponding regulations define the term “postmark.” As a result, the Final Rule effectively supplies a definition in Section 7502(a).
The Final Rule acknowledges that there are many third‑party uses of postmarks as proof of timeliness, including the IRS’s reliance on postmark dates to determine whether a filing is timely. While taxpayers have long assumed that mailing a return at the post office ensures that same day’s postmark, the Final Rule’s clarifications make clear that this is not always true. Because of the recent changes to USPS transportation operations, mailpieces are now more likely to receive a postmark date later than the date they were deposited at the post office.
Key takeaways
To avoid the risk of late filing, taxpayers should carefully consider the new Final Rule when determining whether to mail their tax filings and payments and, if so, what methods of mailing to consider.
For taxpayers wishing to file their taxes via first-class mail, best practices include:
- Mailing early to provide a buffer in case the item is not processed the same day
- Requesting a manual postmark from a USPS retail employee at the time of mailing
- Requesting a postage validation imprint label to document the date the mailpiece was received by USPS
- Purchasing a Certificate of Mailing to document the date the mailpiece was received by USPS.
For taxpayers open to first-class mail alternatives, best practices include:
- Using USPS Registered Mail or USPS Certified Mail with return receipt requested
- Using an IRS-designated private delivery service, which provides its own proof-of-mailing documentation.[2]
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[1] 26 C.F.R. § 301.7502-1(c)(1)(iii).
[2] See 26 C.F.R. § 301.7502-1(c)(3), (e)(2)(ii). The date of delivery to a private delivery service is the date that is recorded in the service’s electronic database and is equivalent to proof of registered or certified mailing through USPS.
