New Regulations Clarify Postmarks and the “Mailbox Rule”

USPS regulations clarify that postmarks may not match the actual mailing date, impacting IRS “timely mailed, timely filed” rules for tax documents.

On Nov. 24, 2025, the United States Postal Service (USPS) issued final regulations, effective Dec. 24, 2025, defining postmarks, identifying the types of USPS markings qualifying as postmarks, and describing the circumstances under which those markings are applied. The regulations also advise customers how to obtain evidence of the date on which the USPS accepts possession of their mailings.

What has changed?

The new regulations make no substantive changes to existing postal operations or postmarking practices. Instead, the regulations are intended to improve the public’s understanding of postmarks and their relationship to the date of mailing. This is critical when applied to various tax filings.

Postmarks applied at originating processing facilities do not necessarily align with the date on which the USPS first accepted possession of a mail piece. For example, the date on postmarks applied at processing facilities represents the date of the first automated processing operation applied at that facility. The machines that apply the postmarks, and the staff who operate those machines, have no way of ascertaining whether that date happens to align with the date on which the USPS initially took possession of any specific mail piece. In other words, postmarks at processing facilities might have a date later than the date the mail piece was first accepted by the USPS. By contrast, the date on a postmark applied at a retail unit indicates the date on which the mail piece was accepted at the retail unit.

Why is a USPS postmark important?

Various federal tax laws and regulations pertaining to certain documents and payments rely on the “timely mailed, timely filed” doctrine (TMTF), which is codified in Internal Revenue Code section 7502. These documents include tax returns, tax refund claims, tax payments, and U.S. Tax Court petitions (“time-sensitive documents”). TMTF provides that if such documents and payments are properly addressed and mailed by the applicable deadline, and the USPS (or other eligible carrier) postmark date is on or before that deadline, the documents and payments will be considered timely filed or paid, even if delivered after the deadline.

How to prove time-sensitive documents were timely mailed:

With the USPS emphasizing in the new regulations that the postmark may not be consistent with the date of mailing, you should follow these steps to prove timely mailing:

• Present the time-sensitive document at a post office retail counter and request that a USPS postmark be applied manually to the item at the time of acceptance;
• Present the time-sensitive document at a post office retail counter and pay for postage at the retail counter. The USPS will print and apply to the item a postage validation imprint (PVI) label that indicates the date on which the USPS accepted that item;
• Present the time-sensitive document at a post office retail counter and buy a Certificate of Mailing, PS Form 3817. The USPS charges a fee for a Certificate of Mailing, but it is much lower than the fee for certified or registered mail service.  A Certificate of Mailing provides basic proof of mailing that shows the date on which the item was mailed, the addressee’s name and mailing address that were shown on the face of the envelope, and how much postage was paid; or
• Present the time-sensitive document at a post office retail counter and send it using the USPS certified or registered mail service. The sender will receive a USPS postmarked receipt that shows the actual mailing date. The use of certified or registered mail service provides the sender with tracking ability, a record of delivery, signature confirmation of delivery, and the option to buy insurance against loss or damage.

Additionally, the IRS has designated certain private delivery services (PDSs), such as FedEx, UPS, and DHL, as eligible to satisfy TMTF. The eligible services are listed in Notice 2026-30 and here: services. This is important because some of the services offered by these PDSs are not eligible (e.g., “DHL Express, FedEx, and UPS are not designated with respect to any type of delivery service not enumerated in this list”). 

Furthermore, for returns and extension requests, you must use one of these designated street addresses. As the IRS explains, “When you mail a return from the post office to the IRS, the address to send the return is often different depending on whether you are sending the return with a payment or without a payment. However, when using a private delivery service to send a return to the IRS, all returns (including those with payments and those without payments) go to one of the street addresses above.”

CBIZ strongly recommends that you use certified mail, return receipt requested, or one of the designated PDS services for time-sensitive documents. Please contact your CBIZ tax professional for more information or with any questions.