39 CFR § 121.0 - Market-dominant product service standards.

---
identifier: "/us/cfr/t39/s121.0"
source: "ecfr"
legal_status: "authoritative_unofficial"
title: "39 CFR § 121.0 - Market-dominant product service standards."
title_number: 39
title_name: "Postal Service"
section_number: "121.0"
section_name: "Market-dominant product service standards."
chapter_name: "UNITED STATES POSTAL SERVICE"
subchapter_number: "C"
subchapter_name: "POST OFFICE SERVICES [DOMESTIC MAIL]"
part_number: "121"
part_name: "SERVICE STANDARDS FOR MARKET-DOMINANT MAIL PRODUCTS"
positive_law: false
currency: "2026-04-05"
last_updated: "2026-04-05"
format_version: "1.1.0"
generator: "[email protected]"
authority: "39 U.S.C. 101, 401, 403, 404, 1001, 3691."
regulatory_source: "90 FR 10869, Feb. 28, 2025, unless otherwise noted."
cfr_part: "121"
---


# 121.0 Market-dominant product service standards.

Service standards in this part are contingent upon proper acceptance before the applicable day-zero Critical Entry Time (CET). Applying the service standards appearing in this part, effective service standards for combinations of 5-digit originating ZIP Codes and 5-digit destinating ZIP Codes can be found in a lookup table at www.usps.com.